sphincter clencher 

It means when something is a tense moment, your ass tightens up. So a 'close call' is called 'puckering your asshole'

"This project is really a sphincter clencher. I don't know if we're going to make it on time."

sphincter clencher by 4tw March 28, 2008

sphincter clincher 

A quip, joke, remark, or action that is offensive enough to make the listener(s) gasp or go silent. When the questionable comment is said, the listeners' ring of muscle around their anus contracts and continues to hold in a tight grasp.

Janet Jackson's wardrobe malfunction and Kramer's (aka Michael Richards) racist tirade are both great examples of a sphincter clincher.

sphincter clincher by Seriously Funny December 16, 2009