1866, shortening of professional (n.). The adjective is first recorded 1915 (in golfing's pro shop).
"a consideration or argument in favor," c. 1400, from Latin pro "on behalf of, in place of, before, for, in exchange for, just as," from PIE root *per- (1) "forward," hence "in front of, before, first, chief." Pro and con is short for pro and contra (c. 1400) "for and against" (Latin pro et contra).
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