A gat...
Old slang, meanging gun. Came from shortening Gattling gun to just gat. Was used during the prohibition era to name any gun, but specifically the thompson submachine gun, aka The Tommy Gun.
Just get that gat out of my face, and everything will be jake.

or

Sorry guys, but no rapper or gangsta came up with the word Gat. Or chopper, for that matter. Unless you're talking about the prohibition era bootleggers type of gangster, that is.