The root of darebák is dar (gift, Greek dóron) from the verb dáti (to give, Latin dare). The development is as follows: dar = gift darmo (darem) = as a gift, i.e without paying, gratis hence the adjective daremný = worthless -> vicious after a dialectical/colloquial change (like in písemný -> písebný): darebný = villainous darebák = villain