A loser as in niceguys finish last.
Hey niceguy didn't Amanda dump you for that carsalesman? laugh out loud
General: Of or pertaining to any guy who is always pleasant and has more girl friends than girlfriends
Specific: THE Niceguy is the archetypal niceguy.
I just met the Niceguy online. He really is a niceguy, he gave me advice on the best place to take that girl I met on a first date.