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.