The term scarlet letter designates a representation of the letter A in scarlet cloth which Hester Prynne, publicly disgraced for committing adultery and giving birth to an illegitimate child, is condemned to wear in The Scarlet Letter, A Romance (Boston: Ticknor, Reed, and Fields, 1850), by the U.S. novelist and short-story writer Nathaniel Hawthorne (1804-1864).

This novel is set in the Puritan Colony of the Massachusetts-Bay in the seventeenth century. I have found a legal text which confirms the existence of the punishment inflicted on Hester Prynne. This text is as follows, from Acts and Laws Passed by the Great and General Court or Assembly of the Province of the Massachusetts-Bay in New-England. Begun and Held at Boston, the Thirtieth Day of May, 1694.—as published in Acts and Laws, Of His Majesties Province of the Massachusetts-Bay, in New-England (Boston. Printed by Bartholomew Green, and John Allen, (Printers to His Excellency the Governour and Council,) […], 1699):

An Act against Adultery and Polygamie.

WHEREAS the Violation of the Marriage Covenant is highly provoking to God, and destructive to Families.
Be it therefore Enacted by the Governour, Council and Representatives, in General Court Assembled, and by the Authority of the same, That if any man be found in Bed with another mans Wife, the Man and Woman so offending being thereof convicted, shall be severely Whipp’d, not exceeding Thirty Stripes, unless it appear upon Tryal that one party was surprized and did not consent; which shall abate the punishment as to such party.
And if any man shall commit Adultery, the Man and Woman that shall be Convicted of such Crime before Their Majesties Justices of Assize and General Goal Delivery, shall be set upon the Gallows by the space of an Hour, with a Rope about their Neck, and the other end cast over the Gallows: And in the way from thence to the Common Goal, shall be severely Whipt, not exceeding Forty Stripes each: Also every person and persons so Offending, shall for ever after wear a Capital A of two inches long, and proportionable bigness, cut out in Cloth of a contrary Colour to their Cloaths, and Sewed upon their upper Garments, on the out side of their Arm, or on their Back, in open view. And if any person or persons, having been Convicted and Sentenced for such Offence, shall at any time be found without their Letter so worn, during their abode in this Province; they shall by Warrant from a Justice of Peace, be forthwith apprehended and ordered to be publickly Whip’d, not exceeding Fifteen Stripes; and so from time to time toties quoties ¹.

¹ Here, the Latin phrase toties quoties means as often as the thing shall happen.

In the following from The Philosophy of Advertising, published in The Evening Mirror (New York City, N.Y.) of Monday 9^th August 1852, the scarlet letter which Hester Prynne is condemned to wear is curiously used as a term of comparison for something rather trivial:

“What is an advertisement?”
“An advertisement is any sign exhibited by one individual to attract the attention of others to the fact that they may greatly benefit themselves, and slightly benefit the advertiser, by one and the same process. Thus a splendid ball dress, setting off advantageously white shoulders and rounded arms, may be regarded as a most decided advertisement. ‘Wanted, a husband,’ is written as intelligibly on the ivory forehead of the would-be enchantress, as if the words were embroidered on her bosom like the scarlet letter in Hawthorne’s powerful romance.”

From its use by Nathaniel Hawthorne, scarlet letter soon came to be used figuratively in the sense of a stigma, a mark of infamy.

These are the earliest figurative uses of scarlet letter that I have found:

1-: From The Charleston Daily Courier (Charleston, South Carolina) of Wednesday 13^th April 1853:

The bill for indemnifying the Catholics and others, whose property was destroyed on the occasion of the burning of the Ursuline convent, at Charlestown, in 1834, came up for consideration in the House of Representatives yesterday. […] All the speakers concurred in the equity of the claim of those who lost property in consequence of the destruction of the convent, and each urged that good policy, good faith, the honor, the pride and fame of Massachusetts, were involved in the matter. Mr. Egan, who is a Catholic, made a most effective speech. […] He did not regard the matter at issue, he said, as one between Catholics and the state, but as a matter between the citizens and the commonwealth; and, though he rose with diffidence, he did not rise as a suppliant. He was anxious that, by this act of magnanimity, Massachusetts—the great and rich state of Massachusetts—might be relieved of the scarlet letter upon her bosom, the brand upon her forehead, and the blemish upon her fame abroad, which had been, through prejudice, allowed to remain too long.

2-: From American Patriotism, a letter in which ‘Amicus Reipublicæ’ denounced U.S. slavery, published in the New Hampshire Statesman (Concord, New Hampshire) of Saturday 3^rd March 1855:

This foul blot is upon our nation as a nation—and as a whole—North, South, East and West. Our minister abroad cannot free himself from it by saying that he comes from a free State and from his heart abhors slavery. He is the Representative of a slave nation—where more souls writhe and pine in horrid, hopeless bondage than the entire number of her inhabitants at the time of her Revolution—a bondage created and established by a system of laws of refined cruelty, and sanctioned by the Constitution—the organic law of the whole country. That country he represents—her dignity, her honor, if she have any, and her rights.
How must the blush of shame unman him when he is told that Slavery is the “Scarlet Letter” that perpetuates our infamy—that “Slavery” should be inscribed upon our banners—that our national eagle should be blotted and removed, and the abject figure of a bowing slave substituted in its stead—that in our Declaration of Independence we say that “all men are created equal,” ² and yet that in our worse than Algerine cruelty we make a poor and wretched slave of every seventh man, woman and child in our country.

² Cf. history of the phrase ‘(but) some — are more equal than others’.