Green’s Dictionary of Slang

lily (the pink) n.

[rhy. sl.; ult. song title Lily the Pink (1968) by Scaffold]

a drink.

2001

K. Sampson Outlaws (ms.) 97: ‘Oi, Bernie, you kant where we going for a Lilly?’ ‘A fucking what?’ ‘A facking Lilly the Pink you slay-agg!’.