Venus Fly Ass
The involuntary response of a heterosexual male when you place a figer too close to his ass hole. The hair detects activity, immediately causing the asshole to tightly clamp shut and the guy in question to jump.
During foreplay, when Mel put her finger too close to Jon's Venus Fly Ass, he clenched and yelled "hey! don't do that" - my ass is a one-way chute!