A 'shag rat' is a man who cheats on his girlfriend with other women. If he is the type to frequently play away from home he is known as a 'serial shag rat'.
e.g.
Girl 1: I heard Alana dumped that bloke she was seeing.
Girl 2: Yeah what a tosser he turned out to be a complete shag rat!
Girl 1: I heard Alana dumped that bloke she was seeing.
Girl 2: Yeah what a tosser he turned out to be a complete shag rat!
by Romany Ravenflame July 05, 2009