Green’s Dictionary of Slang

(Aus./Can.) the lavatory in a bar.

1983		R. Aven-Bray Ridgey-Didge Oz Jack Lang 51: Used Beer Department Hotel toilet.
1987		J. Ciardi Good Words 32: Used beer Urine [...] Any number of U.S. beer joints found a point of high humor in labelling their restrooms ‘Used Beer Department.’.