Green’s Dictionary of Slang

honestly, fairly; usu. in comb. with treat.

1878		H.O. Flipper Colored Cadet at West Point 54: ‘To be white,’ ‘To treat white.’ — To be polite, courteous, and gentlemanly.
1890		Nat. Police Gaz. (NY) 3 May 6/4: ‘We’ve treated you d—d white’.
1897		Worker (Brisbane) 4 Sept. 8/3: A pound a hundred, ‘in or out,’ with rules and tucker fair, / He designates as ‘working white’ and shearing ‘on the square’.
1899–1900		C.L. Cullen Tales of the Ex-Tanks 20: Never got handled so white in my life.
1907		C. M’Govern By Bolo and Krag 50: Well, sir, they treated us white did them gu-gus.
1911		A. Adams Wells Brothers 🌐 Ch. ii: They’ve treated me white, and I’m going to make a fight for them.
1912		Bulletin (Sydney) 12 Dec. 42/3: D’yer think I’d go givin’ it erway like that arter yer treatin’ me white like y’ve done?
1922		Joyce Ulysses 436: I treated you white. I gave you mementoes, smart emerald garters far above your station.
1927		‘Max Brand’ Pleasant Jim 304: You’ve played so white with me.
1933		J. Conroy Disinherited 183: Some o’ you crummy bastards don’t appreciate it when a man aims t’ treat ya white.
1978		R. Price Ladies’ Man (1985) 93: Everybody was treating me white.