con (n.1) Look up con at Dictionary.com
"negation" (mainly in pro and con), 1570s, short for Latin contra "against" (see contra).
con (n.2) Look up con at Dictionary.com
"study," early 15c., from Old English cunnan "to know, know how" (see can (v.1)).
con (adj.) Look up con at Dictionary.com
"swindling," 1889, American English, from confidence man (1849), from the many scams in which the victim is induced to hand over money as a token of confidence. Confidence with a sense of "assurance based on insufficient grounds" dates from 1590s.
con (n.3) Look up con at Dictionary.com
a slang or colloquial shortening of various nouns beginning in con-, such as, from the 19th century, confidant, conundrum, conformist, convict, contract, and from the 20th century, conductor, conservative.
con (v.1) Look up con at Dictionary.com
"to guide ships," 1620s, from French conduire "to conduct, lead, guide" (10c.), from Latin conducere (see conduce). Related: Conned; conning.
con (v.2) Look up con at Dictionary.com
"to swindle," 1896, from con (adj.). Related: Conned; conning.