To hold a shit in for a long time, especially when it's not an appropriate time to take a dump.
I've been clenching ever since I got to work.
A neo-modern activity, involving the act of heterosexual men attending a homosexual nightclub with friends, traditionally for cheap drinks and the prospect of meeting an attractive woman with experimental tendencies. In doing this, the man often has to awkwardly reject the advances of drunk homosexual men, and hence 'clenching' their bumcheeks together.
Can be justified in conversation by shrugging shoulders and saying "YOLO."
"Hey, what are you doing tonight?"
"I'm going clenching with some friends"
"Ha, good luck.."