con (n.1)youdaoicibaDictYouDict[con 词源字典]
"negation" (mainly in pro and con), 1570s, short for Latin contra "against" (see contra).[con etymology, con origin, 英语词源]
pro (n.2)youdaoicibaDictYouDict
"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-and-conyoudaoicibaDictYouDict
"To weigh the arguments for and against; to debate both sides of (a question or matter)", Late 17th cent. Probably partly from pro and con, and partly from pro + and + con.