If a guy says it it means are you trying to fight/do you want to fight but if a girl says it (to a guy) it means do you want me to give you a hand jobbasically
The hand you don't use while masturbating. 'Catch hand' applies when in the slightly unfortunate situation where you masturbate past the point of no return, but suddenly realise that you don't have a tissue. Or a sock. Or a girlfriend!