What does the Greek word “malaka” mean?

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.