A fuck boy is essentially the male equivalent of a slut or being slutty except that it is neither an insult nor compliment. Fuck boys want one thing when talking to girls, sex. Not to be confused with douchebags which are just assholes who try to pick up girls but often fail.
Fuck boy: Hey girl how you doin'
Girl 1: Pretty good how about you?
Girl 2: *whispers* watch out hes a fuck boy