The alliterative phrase prunes and prism, also prunes and prisms, designates a prim or affected facial expression or manner of speaking; also, in extended use, affected mannerisms, superficial accomplishments.

This phrase occurs, for example, in the review of A Christmas Carol, produced at Little Missenden by the Miscasts Dramatic Society—review by Bob Jolowicz, published in the Amersham and Chesham Advertiser (Amersham, Buckinghamshire, England) of Wednesday 2^nd February 1994:

Each of the five cast members played many parts, like the main one, Diane Healy, who carried the role of Mrs Reece, the Guild’s President presumably, in charge of proceedings. Her appearance was more than fortuitously lookalike for Margaret Thatcher, complete with handbag. Her prunes and prisms accent was beautifully created as was her utterly insincere tinkle of laughter when things went wrong.

This phrase originated in Little Dorrit (London: Bradbury & Evans, 1857), by the English novelist Charles Dickens (1812-1870). In this novel, prunes and prism is a phrase spoken aloud in order to form the lips into an attractive shape [cf. note]—it first occurs in Chapter V, titled Something Wrong Somewhere:

“Amy,” said Mr. Dorrit, “you have just now been the subject of some conversation between myself and Mrs. General. We agree that you scarcely seem at home here. Ha—how is this?”
A pause.
“I think, father, I require a little time.”
“Papa is a preferable mode of address,” observed Mrs. General. “Father is rather vulgar, my dear. The word Papa, besides, gives a pretty form to the lips. Papa, potatoes, poultry, prunes, and prism, are all very good words for the lips: especially prunes and prism. You will find it serviceable, in the formation of a demeanor, if you sometimes say to yourself in company—on entering a room, for instance—Papa, potatoes, poultry, prunes and prism, prunes and prism.”

Note: Charles Dickens elaborated on the phrase to say prunes, which means to speak the plural noun prunes aloud in order to form the lips into an attractive shape, and is particularly associated with portrait photography.

The phrase prunes and prism often reoccurs in Little Dorrit. In particular, Chapter VII is titled Mostly, Prunes and Prism, and contains the following passage:

The wholesale amount of Prunes and Prism which Mrs. General infused into the family life […] left but a very small residue of any natural deposit at the bottom of the mixture. […] Prunes and Prism, in a thousand combinations, ha[d] been wearily in the ascendant all day—everything having been surface and varnish, and show without substance.

The phrase prunes and prism is occasionally used attributively in Little Dorrit—the following, for example, is from Chapter XV, titled No Just Cause or Impediment why these Two Persons should not be joined together:

Mrs. General changed her gloves, as to the right glove being uppermost and the left undermost,  with a Prunes and Prism smile.

After quoting the passage from Chapter V of Little Dorrit in which prunes and prism first occurs, the anonymous author of Remonstrance with Dickens, published in Blackwood’s Edinburgh Magazine (Edinburgh: William Blackwood & Sons) of April 1857, criticised Dickens’s repeated use of prunes and prism in this novel, but predicted that it would nevertheless become a household phrase:

Will it be believed that the Dickens whom we remember of yore (eheu, quantum mutatus!), instead of being ashamed of this puerility, becomes positively enamoured of it, as an excellent joke worthy of frequent repetition? […] This gibberish goes on acquiring importance, till, in the author’s mind, it means something, though what, we don’t know. […] It is difficult to perceive by what steps a humour so true and rich as Dickens’s could descend to this, and revel in it. […]
[…]
[…] Dickens, dear Dickens, […] we know that you must of necessity be surrounded by admirers of more enthusiasm than discretion. We know that if you were unhappily afflicted with a brain-fever, and your delirious utterances were taken down in shorthand, and published as a serial, plenty of foolish readers would be found to admire, plenty of foolish critics to applaud. This is only to say that you are a great writer with a vast reputation, and therefore whenever you hold up your finger the multitude will shout. Cry but “Mum,” and thousands of voices will respond with “Budget.” We don’t doubt that your foolish joke about prunes and prism will be bandied from thousands of silly mouths as a household word.

The earliest occurrences of the phrase prunes and prism, also prunes and prisms, that I have found are as follows, in chronological order:

1-: From the review of Captivity of Two Russian Princesses in the Caucasus: Including a Seven Months’ Residence in Shamil’s Seraglio. Communicated by themselves, and translated from the original Russian by H. Sutherland Edwards (London: Smith, Elder, and Co., 1857)—review published in The Examiner (London, England) of Saturday 5^th December 1857 (Verderevsky was the author of the original account in Russian):

At once we see that M. Verderevsky seizes upon the disconsolate French governess as a comic element in his story, and accordingly we find her carried through the book in a tone of polite satire as a sort of Mrs General in difficulties, wasting prunes and prisms on the wild men of the hills.

2-: From the review of The World and the Stage, a comedy produced at the Haymarket Theatre, London, published in The Era (London, England) of Sunday 13^th March 1859:

Mr. W. Farren, the acknowledged representative of the fashionable fraternity endowed with elastic ethics, was gay and animated; Mr. Compton admirable in the imitative Buzzard; Mr. Rogers portly and pompous as the prejudiced Sir Norman Castlecrag; and Mrs. Poynter a perfect model of propriety of the “prunes and prism” school.

3-: From the review of Measure for Measure (London: Hurst and Blackett, Publishers, 1862), a novel by Mrs. A. B. Church—review published in The Morning Post (London, England) of Wednesday 3^rd September 1862:

The course of the narrative runs easily and naturally, from the introduction of Beatrice into the society of a populous country neighbourhood, in which her father has purchased an estate. She makes conquests and friends, she also makes enemies, especially among the proper young ladies, true to a standard of engaging mediocrity and the famous formula of “prunes and prism.”

4-: From Mr. Disraeli’s History and Morality, published in The Northern Whig (Belfast, County Antrim, Ireland) of Tuesday 10^th February 1863—here, prunes and prisms is quoted as a nonsensical alliterative phrase:

His opponents are “prigs and pedants”—evidently not less because both words begin with a “p,” and the alliteration is too attractive to be resisted, because they differ from Mr. Disraeli in opinion. If, like Mrs. General, he had called them “prunes and prisms,” it would have done almost as well.

5-: From a correspondence from London, published in The Dover Express and East Kent Intelligencer (Dover, Kent, England) of Saturday 21^st May 1864:

More perfect holiday weather than we have had for our “outings” it is impossible to conceive: it has been simply glorious. How blithe it must have made the hearts of the poor Sunday School children, who have had another of their too rare treats! What between their respect for authority, as represented by the serious spinster who marches in their rear, and never for an instant takes her eye off the entire squad, and what between the natural impulse of youths to break out of the too stiff ranks and shock all the proprieties of the maiden monitor, I own that some of these juvenile bands hardly seemed to me in their ease, and would have infinitely preferred a romp in which “prunes and prism” had no possible share.

6-: From Keeping Out of It, published in All the Year Round. A Weekly Journal. Conducted by Charles Dickens (London, England) of Saturday 17^th December 1864—the noun rose-rash is a synonym of roseola:

Many people there are in this busy life of ours whose mission seems to be that of perpetual train-bearers to their friends and acquaintances. These are the people always at hand for whatever is wanted. Baby cannot cut its teeth, Jacky cannot have the rose-rash consequent on too much Christmas pudding, and Louisa cannot be invested with her first ball-dress and white satin slippers, without Miss Muchlove’s presence and concurrence. Every event in the family—every birth, and death, and marriage, and change of season with its attendant routing out of wardrobes, every new servant, and every old bonnet—brings up Miss Muchlove from the depths of Camberwell (her titular home) at a vast expense of time, toilette, and omnibus hire; and no one thinks it too much to demand of her all her hours and half her income, to help them to settle the rags and jags of their untidy days. Of course Miss Muchlove might turn crusty if she so willed it; she is not chained and padlocked to subserviency, and friendship is not like marriage, and can be flung overboard when becoming too weighty an inheritance; but, Lord bless that tender heart and soft head of hers! she is as incapable of resisting a request, even the most monstrous, as she is of heading a regiment, and finds No the most difficult monosyllable of the English language. […] If the Miss Muchloves of the world would only learn to say No with half their present facility in saying Yes, there would be fewer trains left trailing in the streets, and fewer train-bearers found to hold them up at their own cost. We teach our daughters to say plums, prunes, and prism. If we would but teach them to say No in the right place, too!

7-: From Miscellaneous, published in The Shepton Mallet Journal (Shepton Mallet, Somerset, England) of Friday 18^th August 1865—the German-born British conductor August Friedrich Manns (1825-1907) was the Musical Director of the Crystal Palace, London, from 1855 to 1901:

Mr. Manns’ Benefit at the Crystal Palace.—The concert which took place on Saturday for the benefit of that intelligent and indefatigable caterer for the musical tastes of the Crystal Palace visitors, Mr. Auguste Manns, was in all respects as great a success as the intrinsic attracions [sic] of the performances, and the very honourable claims of the beneficiare [sic] upon the friendly offices of the metropolitan public deserved that it should be. True, the season, which the dissolution of Parliament has moreover prematurely deranged, is now so far advanced that people who have been brought up with a due regard to the “prunes and prism” aspects of life do not care to have it known that they are still sojourners within any reasonable distance of the suburban beauties of the Sydenham hills; and more sadly true is it that our proverbially contumacious “clerk of the weather,” who has favoured this year’s fashionable fetes with so much bright sunshine as has temporarily removed from English flower-shows the reproach of being but a synonym for thoroughly wet days, has relapsed into his natural capriciousness, and was on Saturday afternoon in a hopelessly lugubrious mood. These disadvantages, however, only served to show how many and how much in earnest were they who desire to acknowledge the benefits which the policy pursued by Mr. Manns ever since he first assumed the conductorial baton has conferred upon those middle classes of London who undoubtedly profit most largely by the delight, both musical and miscellaneous, contained in the Crystal Palace.