Manchild is a nickname we called a guy that was dating a friend's sister.  He was 30 going on 12.  He was a college grad, had an MBA, but didn't do anything but book oil changes part time at a car dealership and play computer games the rest of the time.  He doesn't do anything but mooch off my buddy's sister.
He was also a bit on the chubby side and still wore his 10 year old conference football championship ring that he earned while sitting on the bench the whole game.