Green’s Dictionary of Slang

a bottle, can or glass of beer.

1890		Nat. Police Gaz. (NY) 31 May 2/3: The ghost died, so we had no rocks to buy the foamy at the bar or to rush de growler, either.
1959		L.P. Boone ‘Gator Sl.’ AS XXXIV:2 156: Gators never merely drink; instead, they [...] quaff a foamy or an amber [etc.].
1970		(con. 1950s) H. Junker ‘The Fifties’ in Eisen Age of Rock 2 (1970) 99: A quick brew: quaff a foamy.
2004		D. Clark Apocalyptic Crawfish 16: Buzz [...] slaps Newton sharply on the back. ‘C’mon, Newt, let’s go quaff a foamy.’.