Green’s Dictionary of Slang

(Aus./US) a chilled glass or can of beer.

1961		(con. early 1950s) J. Peacock Valhalla 46: Let’s get some frosties.
1979		B. Gutcheon New Girls (1982) 105: ‘Give my granny a frosty.’ Pear reached into an ice bucket and produced a can of Ballantine ale.
1981		Guardian 27 Mar. 10/4: Ockers are Bazzer Mckenzie type Aussies — all beer gut, shorts and singlets [...] sloshing foaming frosties (ice cold lager).
1994		T.G. Long A Chorus of Witnesses 263: He probably had had a couple of ‘frosties’ too many.
1999		G. Seal Lingo 133: A coldie, referring to a can or bottie chilled in the fridge, the latter also known as frosties and stubbies, not to be confused with the men’s shorts of the same name.