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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 (see pro-). Pro and con is attested from c.1400, short for pro and contra "for and against" (Latin pro et contra).
Pro abbr. proline
: pro ranks
[the last sense perhaps fr professional reinforced by prostitute, or vice versa]
A prophylactic for preventing venereal disease; condom; rubber (WWII armed forces)