Reader KH asks:
I currently have a number of people trying to convince me that the phrase "that's mighty white of you" originated in the American South in the ~1920s, deriving from racial ideas of whiteness and white supremacy. It was my understanding that "white" in this phrase derived from completely non-racial ideas correlating whiteness with purity or goodness. Do you know of any source that might settle this? I have not been able to find anything reliable on my own and thought you might have more extensive resources at your disposal.
I believe that those other people are more right than you are, though they're wrong about the details. The phrase — in its general form "(INTENSIFIER) white of PRONOUN" — seems to have originated in the 1890s, and was not especially associated with the American south. Rather, Americans in general used it to identify behavior felt to be stereotypically associated with WASPs, or at least with those of the better classes, as opposed to the dishonorable behavior to be expected from blacks, indians, jews, and pretty much everybody else.
The OED glosses sense 4.b. of white as
slang or colloq. (by extension from white man n. 2b; orig. U.S.) Honourable; square-dealing. Also as adv.
with these citations:
1876 W. Besant & J. Rice Golden Butterfly II. v. 83 A good fellow is Rayner; as white a man as I ever knew.
1890 Cent. Mag. Feb. 523/2 There ain't a whiter man than Laramie Jack from the Wind River Mountains down to Santa Fe.
1913 E. Wharton Custom of Country ix, Well—this is white of you.
1913 E. Wharton Custom of Country xviii, I meant to act white by you.
It's easy to find citations for the "(INTENSIFIER) white of PRONOUN" usage somewhat before 1913, e.g.
"It's deuced white of you, Vertner," said Philip, with gloomy gratitude; "but you can't do it." (Wolcott Balestier, "Benefits Forgot", The Century Illustrated Magazine, 1893)
(Balestier died in 1891, so Benefits Forgot must have been written earlier than 1893.)
It's worth noting that in those days, "white" didn't have the connotation that it does today, but rather referred (approximately) to northern European protestants and their descendants, excluding (for example) jews:
"Well, I got the wagon all right and one fine, large and subsequent day I pays Mr. Wolf in full, him sorrowfully but firmly declining to partake of any interest; which was pretty damn white of him, when you stop to consider the style of nose he wore, and shows what climate will do." (Eugene Manlove Rhodes, "Sticky Pierce, Diplomat", Out West ["The Nation Back of Us, The World in Front: A Magazine of the Old Pacific and the New"], October 1906)
(For an interesting discussion of the historical transition of Italian immigrants on this dimension, see Jennifer Guglielmo & Salvatore Salerno, Eds., Are Italians White?)
Throughout American history, white was often implicitly or explicitly opposed to indian rather than to black. Thus in James Fenimore Cooper's 1840 novel The Pathfinder,
"A soldier's calling is an honorable calling, provided he has fi't only on the side of right," returned the Pathfinder; "and as the Frenchers are always wrong, and His Sacred Majesty and the these colonies are always right, I take it the serjeant has a quiet conscience, as well as a good character. I have never slept more sweetly than when I have fi't the Mingos, though it is the law with me to fight always like a white man, and never like an Injin."
And in certain places at certain times, the default non-white group would have been Chinese or Mexican.
So the predicative phrase "(INTENSIFIER) white of PRONOUN" seems clearly to be American in origin, but I don't find any evidence that it has any particular association with the American south. In fact, all of the early examples that I've seen are from other parts of the country: the west, the midwest, or the northeast.
It seems to have become common during the period 1890-1920 or so, and to have been used to differentiate the stereotypically dependable ethics of WASPs from the stereotypically questionable ethics of everyone else.
An older British semi-equivalent is the phrase "like a christian", where the noun christian is used in the OED's sense 3.b., glossed as
colloq. or slang. A ‘decent’, ‘respectable’, or ‘presentable’ person.
Typical examples are
1749 H. Fielding Tom Jones IV. xii. iii. 208 A fitter Food for a Horse than a Christian.
1844 Dickens Martin Chuzzlewit xxxiv. 409 You must take your passage like a Christian; at least, as like a Christian as a fore-cabin passenger can.
Similarly, the adjective christian in sense 5.b., glossed as
mod. colloq. or slang. Of things: Becoming a Christian; ‘civilized’, ‘decent’, ‘respectable’.
with examples like
1682 T. D'Urfey Butler's Ghost 76 Christian breeches without hole.
1819 Scott Legend of Montrose vi, in Tales of my Landlord 3rd Ser. IV. 137 Had you been to fight with any christian weapons.
Thus in a passage from William Congreve's The Old Batchelor (1693), Belinda tells Araminta about her attempts in Mrs. Snipwell's shop to improve the appearance of a country squire's wife and two daughters, who were
… so bedeck'd, you would have taken 'em for Friezland hens, with their feathers growing the wrong way — O, such out-landish creatures! Such Tramontanae, and foreigners to the fashion, or any thing in practice! [...]
I did endeavour to make her look like a Christian — and she was sensible of it; for she thank'd me, and gave me two apples, piping hot, out of her under-petticoat pocket.