A member of a sports team (normally the opposition) who is sub-par in some way. Normally used to describe players of an apparently higher or distinctly lower ability lever.
Often invoking images of players who are definitely there for 'a day out', running around with gangly legs, flailing. Shitters will sometimes be partially deaf or semi-retarded
"That kid is a bit of a Shitter"
"Yes, I think I saw him licking the fence on the way in