1824, originally in African-American vernacular in the South.
The slaves themselves entertain the very highest contempt for white servants, whom they designate as 'poor white trash.' [Fanny Kemble, journal, Jan. 6, 1833]
[T]he term [poor white] is rather loosely applied by Northern writers even to mountaineers and to small farmers who live on a precarious footing. But in the Southern conception, not everyone who is both poor and white is a "poor white." To the Southerner, the "poor white" in the strictest sense is a being beyond the pale of even the most generous democratic recognition; in the negro's term, "po' white trash," or so much social débris. [Robert Penn Warren, "The Briar Patch," 1930, footnote]
"person or thing that people hope will be very successful in the near future," 1911, originally in U.S. sporting use in reference to the quest for a white man capable of beating champion pugilist Jack Johnson.
"burdensome charge, inconvenient thing that one does not know how to get rid of," 1851, supposedly from the practice of the King of Siam of presenting one of the sacred albino elephants to a courtier who had fallen from favor; the gift was a great honor, but the proper upkeep of one was ruinously expensive.
as a symbol of cowardice, 1785, said to be from the time when cock-fighting was respectable, and when the strain of game-cock in vogue had no white feathers, so that "having a white feather, is proof he is not of the true game breed" [Grose].
type of long, warm underwear, 1943, originally made for U.S. GIs in World War II. Earlier as the name of a type of pastry (1919). Also used of sorts of worm, potato, table, sled, etc.
1707, "paper duly authenticated by a signature and otherwise blank, left to someone to be filled in at his discretion," French, literally "white paper," from carte (see card (n.1)) + blanche, from Old French blanc "white," a word of Germanic origin (see blank (adj.)). The figurative sense of "full discretionary power, unrestricted permission or authority in some manner" is from 1766. Compare the native blank check, used in the same figurative sense.