Green’s Dictionary of Slang

1. (Aus.) an eight ounce beer glass; a serving of beer in such a glass.

1972		J. O’Grady It’s Your Shout, Mate! 56: ‘What do we call them? Fours, sixes and eights’.
1977		Baglin & Austin Australian Pub Crawl 126: In Tasmania beer glasses are logically called Four, Six, Eight and Ten according to their relative ounces.
2000		P. Brown Three Sheets To The Wind 241: Once you’ve mastered these technicalities, you might want to try ordering a [...] ten, eight or six, a handle, a seven, a butcher, or simply a glass - or perhaps a small glass.
2012		G. Ellis Australian Bar Attendant’s Handbook 33: Tasmania: [...] Eight: 225 ml: 8 oz.

2. (US drugs) an eighth of an ounce of a narcotic, usu. heroin.

1974

V.E. Smith Jones Men 123: H’s only been handling eights up to now.

3. (US drugs) heroin [H is the eighth letter of the alphabet].

1979

J. Homer Jargon.

4. (US black) a .38 calibre handgun.

1991		Schoolly D ‘King of New York’ 🎵 Grab the uzi, the eight and the nine.
2002		Lil Wayne ‘Bloodline’ 🎵 My palm is wrapped around this eight.
2016		Lil Reese ‘Jakes’ 🎵 Get the fuck out my face, before I hit you with the eight.