1. A guy who joins a gym but doesn't like to do anything outdoors.
2. Someone into an indoor sport like bowling or darts.
3. A guy who says he's an athlete to impress women, but they're so skinny/pale you know they never go outside.
4. A wanker.
(Def 2) I blew out my knee playing basketball, now I'm an indoor sportsman.
(Def 3 and 4)
Guy 1: So I was talking up this girl, right, she was checking out my biceps ...
Guy 2: What, who, you? The indoor sportsman? Did you show her your "trophy"?