A.K.A. Unfinished Sexual Business. The crushing psychological weight which arises when one doesn't use a sexual opportunity to one's full advantage. A kiss is nice, but sometimes it just doesn't cut it and you'll always wonder what could have been. USB is often stronger during a sexual drought.
When it gets to a few months after you last shagged and you're wondering where your next meal's coming from, then you'll get some serious USB, and then I'll remind you how stupid you were to turn that girl down.