Offside means a player has to have two opposing players between them and the opponents' goal the moment a ball is forwarded. The offside trap is when you intentionally cause your opponent to be offside by stepping back so there's only one player between them and the goal.

You can see the best offside trap ever here: https://www.youtube.com/watch?v=8RIbPfWMNJc it's pretty neat to watch.

I think if you want to say "fuck me" in football, you could say "9-0-1 me" though!