Green’s Dictionary of Slang

(UK Und.) a gentlemen’s public convenience.

1937

J. Worby Other Half 184: ‘Better drive round till we come to an all-night cozzy’ [...] ‘Yes,’ he said. ‘I think I could do one too.’ We soon arrived at the cozzy and the Brains said: ‘You go in first and I’ll wait in the tub.’.