A worthless spineless guy.
One who abuses insecurities of girls to gain sexual pleasure at their expense; the act of acting on a girls insecurity (emotional or otherwise).
In reference to the above - one that maintains a relationship purely to obtain his partner sexually by any means necessary (lying about living her etc...)
A guy that cheats on his girlfriend (who stupidly believes in him through an act of manipulation of her weaknesses), who then finds out about it but still believes that he loves her.
Guy with a small dick.
'that guy jonnymansoned my sister'
'why is she really upset? she realised her boyfriend is a jonnymanson. she took it really hard'
'How big is your boyfriends dick? About 10 jonnymansons'