Someone who is
a) irritating you
b) starting an argument
c) called marnie

When a women watches either a really scary film or a bit in a film which has a tense or climatic part to it, tends to clench her fanny together tight from excitment and nerves. The women may also squeeze her thighs tighly and also her asre cheeks too.

When you feel as if you are about to break wind and just as you begin to attempt to pass it, you feel a sudden moist feeling on the edge of your butt cheeks. At this point you come to realize this may not be what you first thought it was and not wanting to turn the back of your legs into a chocolate waterfall, You snap at attention. Immediately clenching your buttcheeks tightly together to avoid any spillage and keeping a firm rigid stance, like a Buckingham Guard. With a cold sweat beginning to form and the feeling of a sharp stabbing sensation in your lower abdomen, you must continue to stay in this position until the pain subsides and you can get to a toilet.

Modern self defense tactic in which the person clenches their anus and moves abruptly to break or neutralize any object inside of it

When something makes you so nervous or scared, it makes your butt clench

A painful blow job that is the result of the giver incorporating too much teeth into their bobbing of the knob. This painful teething of the cock causes the male to clench their own teeth and often grimace in pain. However, the male receiving will generally endure the pain because even though it hurts, any sucking of the cock is good.

When a girl gets an involuntary twinge in the groin region as a result of thinking of/ witnessing something arousing.