Green’s Dictionary of Slang

1. a glass of sherry taken as the finale after a meal spent drinking port and claret [its relatively ‘white’ colour and its ‘washing’ away of the red wines].

1854		G.J. Whyte-Melville General Bounce 338: ‘Just three words, Charlie,’ said the General, sipping his sherry; ‘won’t you have a whitewash, my boy?’.
1864, 1867, 1870		Hotten Sl. Dict.
1904		R.C. Lehmann Conversational Hints 147: I don’t mind if I do have a chunk of cake and a whitewash of sherry.

2. (also wash) in sport, the complete defeat of one team by another; thus any crushing defeat; also attrib.

1867		Ball Players’ Chronicle 20 June 4/3: A blank score at Albany, N.Y., is called ‘a blind;’ in Connecticut it is a ‘white wash’ [DA].
1886		Lantern (N.O.) 29 Sept. 3: [He] saved his side from a wash.
1906		L.A. Herald 27 Apr. 6/1: Shutouts appear the order nowadays and yeserday’s whitewash was smeared over the Angels, who succumbed [...] to the tune of 4-0.
1929		Western Argus (Kalgoorlie, WA) 20 Aug. 6/2: The Multnomah baseball team [...] registered the second whitewash of its tour when it shut out the Victorian State side.
1946		Oregonian 22 July II 1/4: Ferriss’ fine job was his fifth whitewash trick of the season and his fifteenth victory [DA].

3. (US) milk.

1945

F.H. Hubbard Railroad Avenue 367: Whitewash – Milk.