Slang referring to marijuana. Term originated in the midwest, and is used commonly in private high schools as a code word, usually as a substitute for the word "boof".
Mostly to describe 'regs' or middies - but certainly acceptable to describe a bustedbroad. Other than weed and punan, bunk can be used to describe anything that sucks.
I wouldn't hit that with a ten foot pole - she be bunk as hell.