Green’s Dictionary of Slang

whitewasher n.

[var. whitewash n. (1); one ‘cleans up’ the palate]

a glass of white wine taken at the end of dinner.

[Aus]‘Rolf Boldrewood’ Colonial Reformer III 144: A whitewasher, Croker? I really believe this is a bottle of the Manzanares you prefer, and we’ll join the ladies.