pro (n.1) Look up pro at Dictionary.com
1866, shortening of professional (n.). The adjective is first recorded 1915 (in golfing's pro shop).
pro (n.2) Look up pro at Dictionary.com
"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).