once-a-weeker
Somebody who is very inexperienced at a sport. When somebody who plays a sport 24-7 and messes up, can be called a once-a-weeker meaning a scrub.
Tennis player-'Hits shot in the net'
Coach-'Dont miss that shot. Your such a once-a-weeker.'
Coach-'Dont miss that shot. Your such a once-a-weeker.'