- word-forming element meaning "forward, forth, toward the front" (e.g. proclaim, proceed); "beforehand, in advance" (prohibit, provide); "taking care of" (procure); "in place of, on behalf of" (proconsul, pronoun); from Latin pro "on behalf of, in place of, before, for, in exchange for, just as," which also was used as a prefix.
Also in some cases from cognate Greek pro "before, in front of, sooner," which also was used in Greek as a prefix (e.g. problem). Both the Latin and Greek words are from PIE *pro- (cf. Sanskrit pra- "before, forward, forth;" Gothic faura "before," Old English fore "before, for, on account of," fram "forward, from;" Old Irish roar "enough"), extended form of root *per- (1) "forward, through" (see per).
The common modern sense "in favor of, favoring" (e.g. pro-independence, pro-fluoridation, pro-Soviet) was not in classical Latin and is attested in English from early 19c.