A combination of the word Hash and Abstinent.
A person who is avoiding any use of cannabis and it's illegal products: Marijuana, Hashish, Ganja, etc.
Mostly a person who used to consume Cannabis, but stopped for personal reasons, like trauma.
1) "Hey man, we smoked that crazy Kush and we still got some left, do ya wanna get wasted?"
"Naw man, i'm Hashstinent since my hoe passed out"
2) "Hey, why is Jim being gay? he just studies all day!"