1. a "chopped" motorcycle. From the practice of cutting parts off and otherwise modifying a stock motorcycle to customize it and improve its performance
2. an old military term for a machine gun. It was not made up by some rapper. The term goes back decades, and was used in WW2.
"what do you drive?"
"a chopped down 1984 Harley, you?"
"nice, I sold my chopper when I had kids, now I drive a mini van"
Dolan: "You know the orders. No fraternizing."
Alfie: "Who's fraternizing? I only asked if he's got bullets in that chopper. I don't want him to take his play-acting too much to heart."
(from the 1967 movie Tobruk)