If a guy's a slut and especially when he brags about it, he can be called a dirty dawg.
An underdawg is a guy who's a slut but keeps it on the down-lo.
A: "Corr that guy's a bit fit... I bet he's a dawg"
B: "Yeah but don't worry he's and Underdawg, you can get with him and you won't have to worry about him spreading it."