In British English, Jenny Darby, Johnny Darm, and variants, were originally opprobrious names for any member of the new Metropolitan Police introduced in 1829 by the Home Secretary Robert Peel (1788-1850).
The British Parliament website explains the following about An Act for improving the Police in and near the Metropolis, known as the Metropolitan Police Act, which was passed in 1829:
The new Act established a full-time, professional and centrally-organised police force for the greater London area under the control of the Home Secretary. The uniformed constables embodied a new style of policing in contrast to the small and disorganised parish forces of the 18th century.
—Cf. also origin of ‘bobby’ and ‘peeler’ (police officer).
The following about the establishment of this new police is from Mysteries of Police and Crime: A General Survey of Wrongdoing and its Pursuit (New York: G. P. Putnam’s Sons – London: Cassell and Company, Limited – 1899), by the British military officer, prison administrator and author Arthur Griffiths (1838-1908):
The scheme of an improved police was denounced as a determination to enslave, an insidious attempt to dragoon and tyrannise over the people. Police spies armed with extraordinary authority were to harass and dog the steps of peaceable citizens, to enter their houses, making domiciliary visitations, exercising the right of search on any small pretence or trumped-up story. There were idiots who actually accused the Duke 1 of a dark design to seize supreme power and usurp the throne; it was with this base desire that he had raised this new “standing army” of drilled and uniformed policemen, under Government, and independent of local ratepayers’ control. The appointment of a military officer, Colonel Rowan, of the Irish Constabulary, betrayed the intention of creating a “veritable gendarmerie.” The popular aversion to the whole scheme, fanned into flame by these silly protests, burst out in abusive epithets applied to the new tyrants. Such names as “raw lobsters” from their blue coats, “bobbies” from Sir Robert Peel, and “peelers” with the same derivation, “crushers” from their heavy-footed interference with the liberty of the subject, “coppers” because they “copped” or captured his Majesty’s lieges, survive to show contemporary feeling.
1 The Duke of Wellington was the head of the Administration when Peel brought forward his measure in 1829.
Jenny Darby, Johnny Darm, and variants, are alterations of the noun gendarme (denoting a paramilitary police officer in France), with full or partial folk-etymological remodelling variously after the female forename Jenny, the male forename Johnny, and the surname Darby. This was mentioned in the review of Folk Etymology: A Dictionary of Verbal Corruptions or Words Perverted in Form or Meaning, by False Derivation or Mistaken Analogy (London: George Bell and Sons, 1882) by the Reverend Abram Smythe Palmer (1844-1917)—review published in The Pall Mall Gazette: An Evening Newspaper and Review (London, England) of Friday 22nd December 1882:
Gensdarmes has been corrupted into the English slang name Johnny-Darbies, now applied as a nickname to policemen, side by side with the classical Bobby, the dignified Robert, and the historical Peeler.
The earliest occurrences that I have found are, as Jenny Darby, from a story published in several British newspapers on Tuesday 10th August 1830—for example in The Sun (London, England):
A big fellow, who turned out to be a watchman, belonging to Broad-street ward, was brought before the Lord Mayor, charged with rioting in the streets and breaking the windows of […] Mr. Ward, a carver and gilder in Bishopsgate-street […]. Mr. Ward […] called an officer, to whom he complained of the conduct of the watchman. The officer instantly seized the warlike watchman, and requested that he would quietly march before the Lord Mayor. “No,” said the watchman, “I’m blow’d if I do. I don’t want to see no Lord Mayor,” and he thereupon called aloud to the crowd, “Here’s the — Jenny Darbys; we shall be pulled and destroyed by the Jenny Darbys.”—(Laughter.)
The Lord Mayor begged to know from the officer what he meant by Jenny Darbys.—The officer said the defendant meant the French police, and although he called them by their French name, the Jenny Darbys, it was wonderful how the common people, what knowed little or no French, understood his meaning.
The Lord Mayor—Aye, it was a serious word to use.
The officer said it certainly was a dangerous word, for the moment the defendant called out, the mob rescued him, and it was with difficulty he (the officer) again got hold of the unruly fellow.
The Lord Mayor—What! did nobody go to your assisiance [sic]?—The officer said, “Not a soul, my Lord; the minute he called me a Jenny Darby, all persons cleared away from me, and we was left alone, arter I took him the second time, and then he refused to walk. He would, as he was going to the Mansion-house, go like a gentleman, and so we was obligated to get him a coach.”
On Sunday 15th August 1830, The Age (London, England) reprinted the same story so as to attack the new Metropolitan Police:
JENNY DARBYS.—This is a very agreeable little bit of City French, and we positively have a mind to introduce it westward. The Things are getting every day so obnoxious, that it is with difficulty any name can be found sufficiently degrading. The feeling against their establishment in the City is so strong, that the very cry of “Jenny Darby” sounds like a tocsin in the ears of the multitude. The other day a ragged lover of the Constitution was brought before the “City Article,” charged with shouting the fearful words:—
The Lord Mayor begged to know from the officer what was meant by Jenny Darbys.
The officer said the defendant meant the French police, and although he called them by their French name, the Jenny Darbys, it was wonderful how the common people, who knowed little or no French, understood his meaning.
The Lord Mayor—Ay, it was a serious word to use.
The officer said it certainly was a dangerous word, for the moment the defendant called out, the mob rescued him, and it was with difficulty he (the officer) again got bold of the unruly fellow.
The Lord Mayor—What! did nobody go to your assistance?
The officer said—“Not a soul, my Lord.”
Have we not seen of late that the Gensd’armes, or Jenny Darbys, of France were the creatures of the Minister? Is there nothing to apprehend at home—is there no possibility of danger?
Likewise, the following drawing—in which Johnny Darmy occurs—is an attack on the Metropolitan Police; it is one of the drawings that were published under the title of The Gallery of 140 Comicalities, Which has appeard [sic] from time to time, in that most Popular Sporting Sunday Paper, “Bell’s Life in London.” (London: George Goodger, 24th June 1831):
THE DEVIL AMONG THE RAW LOBSTERS.
“I’m an Englishman, bred and born, and I sharnt submit to any o’ your Johnny-Darmies without knowing the why and the wherefore. I say you are an unconstitutional force, and no more to compare to the old Charlies 2 than Bob Peel is to Joe Hume 3!”
2 Charley, also Charlie, was the name formerly given to a night-watchman.
3 Joseph Hume (1777-1855) was a British Radical politician.