step-manchild

A male between the ages of 19 and 31 that is usually the boyfriend of a single mom. They are usually too immature to act as a role model and have a tendency to either be too inept to help watch children and do other parent stuff, thus becoming a burden; or will beat/kill a child over something that's not a big deal, for example, a video game.

Person no.1: did you see her boyfriend
Person no.2: yeah, dude looks too much like he could be a step-manchild