(noun) an extremely painful erection, frequently obtained after a rough night of sexual intercourse. Often irritated by the zipper or rough fabric of a trouser.
Bro 1: Why do you look so pained, friend?
Bro 2: It's this thwamper I can't shake. This thing has been rubbin on my zipper all day!
Wife: Do you want to go again tonight honey?
Husband: I don't know sugar, it looks like tonight's gonna be a thwamper.