To elaborate on the other answers, malakas does indeed mean “masturbator”, but note that it does not have the same connotation as either American jerk < jerk off or Commonwealth wanker. A jerk and a wanker are both obnoxious, presumably because masturbation is narcissistic. A malakas is a fool, a dupe. (Cartoons will often feature J Random Citizen asking themselves whether they are a malakas for voting for party X, or falling for political pledge Y.)
EDIT: by way of illustration (and because this answer, disappointingly, appears to be one of my most popular), here’s a recent instance that popped up on my Facebook feed:
Jerking off makes you blind. [SYRIZA logo.]
The joke is not that voting for SYRIZA makes you a jerk (US slang) or a wanker (Commonwealth slang)—but a fool (Greek slang), for believing that voting for SYRIZA would make any difference.