From Online Etymology Dictionary :
prefix meaning "before, forward, in favor of, in place of," from L. pro "on behalf of, in place of, before, for," also in some cases from cognate Gk. pro "before, in front of," both from PIE *pro-, extended form of base *por- "forward, through" (cf. Skt. para "beyond," pra- "before, forward, forth;" Gk. paros "before," para- "from beside, against, beyond;" Goth. faura "before," O.E. fore "before, for, on account of," fram "forward, from"). Pro and con is attested from c.1400, short for pro and contra "for and against" (L. pro et contra).