Green’s Dictionary of Slang

wampo n.

[? RAF sl. wampo, intoxicating liquor; ? note Wampole, a preparation of alcohol, quinine, and strychnine sold by an eponymous US drug firm c. 1910]

beer slops or overflow, recycled and served as fresh beer.

[UK]Partridge DSUE (8th edn) 1306: [...] from ca. 1930.