When a couple has been dating a while, some tend to get, uh, comfortable. Really comfortable, as in, the girl starts growing out the carpet downstairs (along with putting on ten pounds and a pair of sweat pants).
So, a guy tends to meet a girl, and she's all stoked on bikini waxes and the like, so naturally he thinks he's found himself a winner, so they date a while. But low and behold, after after a few months, Chubakah takes over.
A premuff is a contract (like the prenup) that binds the girl, at signing, to a minimum level of hygiene.
Girl: You know Tom, its been a while now and, since we're exclusive and all, I don't see why we can't call each other girlfriend and boyfriend
Guy: That sounds great, but before we do that, I want us to go over this contract together - its not that I don't trust you, honey I know you'll keep shaving, but its just that I've heard horror stories, will you sign it just for my own reassurance?
Guy just ties the knot = Relationship: Dude, Sarah and I are official, finally! Man, I'm stoked bro, we're gonna do everything together, and we're even talking about moving in!
Friend of guy: That's awesome! You guys really seem happy together. But dude, you signed a premuff right?
Guy that just tied the knot: Nah man, I trust her, she ain't gonna go all Chubakah on me.
Friend of guy: Duuuuuude, you fucked up man, watch, six months in she's gonna grow it out and wondering why you don't love her no more. Damn dude, always have em sign the premuff.