A "threadstomper" is one of the many message board
A "threadstomper" is a user who posts to any thread running, usually useless or unnecessary remarks, for the sake of posting. Usually their posts are generally ignored and remain unreplied to. They are generally unaware of how pathetic they are.
The name derives from the idea that the user is stomping all over a particular thread, leaving an unwanted mess (post).
The term is generally used by either annoyed users ("God, another one of those threadstompers!") or by the threadstompers themselves ("God, I'll never be popular, I'm just a pathetic threadstomper!") upon realising their true forum identity.