The phrase holier-than-thou means: self-righteously or sanctimoniously virtuous, or professing to be so.

This phrase occurs, for example, in The populist right is regretting its encouragement of Covid conspiracists, by Paolo Gerbaudo, sociologist at King’s College London, published in The Guardian (London and Manchester, England) of Wednesday 20^th October 2021:

The Manichean frame of a quasi-religious battle between good and evil that characterises the culture war approach means that any act of moderation or compromise on the part of existing populist—or simply opportunistically populist—leaders can be easily presented as betrayal, opening the space to holier-than-thou challengers, thus splitting the vote.

The phrase holier-than-thou alludes to the Book of Isaiah, 65:5. God Himself is speaking in The Book of Isaiah, 65:1-5, which is as follows in the King James Bible (1611):

1 I am sought of them that asked not for me: I am found of them that sought me not: I said, Behold me, behold me, vnto a nation that was not called by my name.
2 I haue spread out my hands all the day vnto a rebellious people, which walketh in a way that was not good, after their owne thoughts:
3 A people that prouoketh mee to anger continually to my face, that sacrificeth in gardens, and burneth incense vpon altars of bricke:
4 Which remaine among the graues, and lodge in the monuments, which eate swines flesh, and broth of abominable things is in their vessels:
5 Which say; Stand by thy selfe, come not neere to me; for I am holier then thou: these are a smoke in my nose, a fire that burneth all the day.

The earliest occurrences of the phrase holier-than-thou that I have found are as follows, in chronological order:

1-: From The Standard (London, England) of Friday 1^st August 1834:

Mr. Wilkes, Mr. O’Connell, and half-a-dozen other liberal members of parliament, with a few of the third and fourth ranks of the inhabitants of Islington and Pentonville, entertained Mr. T. Duncombe ¹, at the White Conduit Tavern, yesterday. Mr. Wilkes, Mr. Duncombe, and Mr. O’Connell were the principal speakers. Mr. Wilkes made a rather neat apologetical speech in giving the King’s health; he, moreover, made amends for the unavoidable formality by reminding his disloyal hearers, that there are just now a good many dethroned Sovereigns, whose condition must present a wholesome warning to King William. The hon. member for Boston made some facetious allusions to Mr. Duncombe’s imputed frailties, which, if becoming in any one, were certainly least becoming in a gentleman who places himself, or is placed by others at the head of “the holier-than-thou-class.”

¹ This refers to the British politician Thomas Slingsby Duncombe (1796-1861), who was then Member of Parliament for Finsbury.

2-: From The Patriot (London, England) of Thursday 22^nd December 1836:

An Exclusive Church “Plan” is in progress “for a Charitable Institution in Kentish-town,” particulars of which are stated in a circular by the church minister, who by it proposes that “Kentish-town shall be divided into districts, with one visiting lady to each; who may add another as an assistant, with the approbation of the committee. These ladies shall communicate with the minister and the committee, and not come into contact with each other. There shall be no ladies’ committees, and no fixed working parties: it being the object, as far as possible, to encourage ladies to be keepers at home, and not to depart from the reserve and delicacy of their sex, by becoming gadders from house to house.” In the same spirit the circular states, that “Bibles, Testaments, Prayer Books, and Tracts, (selected by the minister of Kentish-town,) shall be distributed to the visiting ladies, who shall also be supplied with small sums of money, increased from time to time, for the comfort of the sick and necessitous whom they may find. They shall read the Bible, with suitable printed prayers, and leave the tracts in the cottages; but no terrifying exhortations—no miraculous conversions—no assurance tests—in short, nothing of a fanatical character, shall be sanctioned.” To support this, and more of the like sort, subscriptions are required to be sent to the vicarage. The Prospectus concludes, and is subscribed, as follows:—“This is the plan which I have drawn up, and which I now submit to the inhabitants. Johnson Grant, M.A., Minister of Kentish-town.” The little blossom thus put forth from a “holier-than-thou” limb of the Church, largely emits the “odour of sanctity,” which, according to the Quarterly, is the only sort fit for use, no other being genuine.

3-: From the account of a vestry “held for the united parishes of All Saints, St. John, and the Liberty of Brickendon and Little Amwell”, published in The Reformer (Hertford, Hertfordshire, England) of Tuesday 20^th June 1837:

It never seems to enter the brains of such wiseacres as the writer in the County Press, that people who do not uphold every abuse of the Church, can be Churchmen at all, or in fact much better than Infidels; we are sorry to say that there are a number of such charitable Churchmen at Hertford, who are continually parading their piety and acting upon the “I am holier than thou” principle.

4 & 5-: From The Chester Chronicle, and Cheshire and North Wales Advertiser (Chester, Cheshire, England):

4-: Of Friday 21^st July 1837:

The Cumberland or High-Church Party is the great source of the animosity and discord which the Queen ², from a Christian and womanly sentiment is “anxious to compose and allay.” But in our own neighbourhood we regret to say Christian Charity has much reason to fear the hostility of the “holier-than-thou-men.” The Cause of the Queen and Reform everywhere encounters their secret or open opposition under the guise of zeal for Protestantism.

² This refers to Victoria (24^th May 1819 – 22^nd January 1901), Queen of the United Kingdom of Great Britain and Ireland from 20^th June 1837 until her death, and Empress of India from 1^st May 1876 until her death.

5-: Of Friday 25^th August 1837:

The “beauty of holiness” is never more beautiful than when suffering under persecution. The good Bishop Stanley ³, the representative of liberty and true Christianity in the Sanhedrim of the Pharisees at Norwich, “disputing with the doctors” is sure of the sympathy of all who read his installation, but what shall be said of “The Reverend Lord Bayning,” ⁴ the holier-than-thou-get-thee-behind-me censor,—what shall we say of him,—to whom shall we compare him, since sacred history does not record the name of the “Chief among the Pharisees who sought to entrap him?”

³ This refers to Edward Stanley (1779-1849), who had just been appointed Bishop of Norwich.
⁴ This refers to Henry William-Powlett (1797-1866), 3^rd Baron Bayning, who was then Rector of Brome, Suffolk.

6-: From the account of the debates that took place in the House of Commons on Friday 15^th February 1839, published in The Somerset County Gazette (Taunton, Somerset, England) of Saturday 23^rd February 1839:

Lord John Russell’s motion, for leave to bring in a Bill to carry into effect the Fourth Report of the Ecclesiastical Commissioners, occasioned one of those debates which usually occur when the Church is before the House, and which, above all other details, are deficient in sense and temper. Ministers are always on such occasions reproached by the Holier-than-thou party with meaning prodigious mischief: we wish they had the courage. When public men are ashamed of their hypocrisy, and publicly say what they privately express, a great reform indeed will be made in the Church.

7-: From The Sun (London, England) of Tuesday 29^th October 1839:

A woman was carried before the Magistrate for selling fruit on the Sabbath; she pleaded that she could get no work, and that she had a husband sick in bed; she was told that it was unlawful to support her husband, unlawful to maintain herself, and that if she persisted in such unlawful, however kindly, affectionate, and honest labours, the Magistrate would enforce against her the strict letter of the law by way of example. He means by punishing Mary Mullooney, to warn all wives that it is unlawful to support themselves and their sick husbands. […]
[…] There is not one merchant in the whole metropolis, not one man of business, whether attorney or solicitor, who would hesitate to follow his worldly calling on that day, not merely if he could get, like Mary, a meal’s victuals for a sick and starving partner, but if he could only add some particular sum to his store, or make an advantageous bargain on that day and not on another. Poor Mary! What the great may do with impunity thou art stricken for by the law; and all affection, all duty, all respect for thy sacred marriage vows must be driven out of thy heart, lest the sanctified, the uncharitable, holier-than-thou women and men, should be offended the exercise of thy mild and wife-like virtues.