A clemin is the generic insult when you don't have an insult. Originally on the television show Family Guy, described as Americas hottest new swearword, but a proper definition was never given on the episode itself.
Often used by idiots and other people who can't form a proper insult. However if used well it can become quite humorous.
In essence it is an ambiguous swear word that is hard to define with a singular definition.
Bob: Hey Tim, I heard that your a dirty homo!
Tim: I am not, I quite like girls.
Bob: Well how come you've never had a girlfriend, and seem to not react to hot girls?
Tim: .... Shutup you.... clemin.
Bob: Oh very good you dirty little retard.