Green’s Dictionary of Slang

1. (W.I.) insincerely accepting religious conversion when the alternative is to be killed [the black man or woman is ‘washed white’ through baptism].

1740

Importance of Jam. 26: Sir Nicholas Laws sent his Letter [...] to demand them [i.e. pirates] of the Spaniards, who answer’d, ‘They were there as other Subjects of their Lord the King, being brought voluntarily over to the Holy Catholic Faith, and have received the Water of Baptism’; which those Rogues nick-name white-washing. So you may guess what sort of Converts they were, and what Credit they brought to their new Religion [DJE].

2. declaring oneself bankrupt (and serving the concomitant prison sentence) to nullify one’s debts; thus whitewashing buck, one who makes such a declaration.

1821		Egan Life in London (1830) 63: The White-washing Buck9 [note 9] A Hero, (belonging to a most numerous family, in 1820,) who gets unblushingly rid of all his difficulties by a bill of indemnity, granted to him for his servitude on board of ‘The Fleet,’ for the short space of three months!
1821		Egan Life in London (1869) 90: The ‘Whitewashing Buck,’ trying on his ‘ways and means’ with the alluring sounds of a Nightingale. [Ibid.] 191: If anything goes wrong with his styling pursuits, a temporary absence from his friends, united with the aid of white-washing, will soon make him ‘all right’ again.
1821		‘An Amateur’ Real Life in London I 47: Not having the courage to pollute his character by a jail-delivery, or to condescend to white-washing, or some low bankrupt trick, to extricate himself from difficulty.
1839		Crim.-Con. Gaz. 24 Aug. 271/1: [T]hat’s the way the money goes. That, old boy, accounts a little for the saeveral whitewashings you have had.
1848		G.W.M. Reynolds Mysteries of London II (2nd Ser.) 32: Fixing his eyes severely upon the youthful candidate for the process of white-washing.