When you realize you have done something so embarrassing or compromising, your whole body seizes, you squeeze your eyes shut, clench fists and especially clench buttocks and imagine that your sphincter has snapped closed.
1.You are mocking your boss and he walks up behind you unbeknown. You're in buttlock.
2. You have farted noisily in the car after you think everybody has already jumped out at the mall only to realize that your future British father-in-law is still sitting quietly behind you, that is until he clears his throat. Ahem. Welcome to Buttlock.
While doing a chick, to put your thumbs on each side of her hip, while your 8 fingers reach over, grabbing her ass with such intensity it causes a "buttlock"