Green’s Dictionary of Slang

anything one can drink.

1861

(con. 1840s–50s) H. Mayhew London Labour and London Poor III 139/2: All beer, brandy, water, or soup, are ‘beware’. [...] This, for instance, is slang for ‘Give me a glass of beer,’ – ‘Your nabs sparkle my nabs,’ ‘a drop of beware’.