Source : Webster's Revised Unabridged Dictionary (1913)

Con \Con\, adv. [Abbrev. from L. contra against.]
   Against the affirmative side; in opposition; on the negative
   side; -- The antithesis of pro, and usually in connection
   with it. See {Pro}.

Con \Con\, v. t. [imp. & p. p. {Conned}; p. pr. & vb. n.
   {Conning}.] [AS. cunnan to know, be able, and (derived from
   this) cunnian to try, test. See {Can}, v. t. & i.]
   1. To know; to understand; to acknowledge. [Obs.]

            Of muses, Hobbinol, I con no skill.   --Spenser.

            They say they con to heaven the highway. --Spenser.

   2. To study in order to know; to peruse; to learn; to commit
      to memory; to regard studiously.

            Fixedly did look Upon the muddy waters which he
            conned As if he had been reading in a book.
                                                  --Wordsworth.

            I did not come into Parliament to con my lesson.
                                                  --Burke.

   {To con answer}, to be able to answer. [Obs.]

   {To con thanks}, to thank; to acknowledge obligation. [Obs.]
      --Shak.

Con \Con\, v. t. [See {Cond}.] (Naut.)
   To conduct, or superintend the steering of (a vessel); to
   watch the course of (a vessel) and direct the helmsman how to
   steer.

Source : WordNet®

con
     n 1: an argument opposed to a proposal [ant: {pro}]
     2: a person serving a sentence in a jail or prison [syn: {convict},
         {inmate}, {jailbird}, {gaolbird}]
     3: a swindle in which you cheat at gambling or persuade a
        person to buy worthless property [syn: {bunco}, {bunco
        game}, {bunko}, {bunko game}, {confidence trick}, {confidence
        game}, {con game}, {gyp}, {hustle}, {sting}, {flimflam}]
     [also: {conning}, {conned}]

con
     adv : on the negative side; "much was written pro and con" [syn: {in
           opposition}] [ant: {pro}]
     [also: {conning}, {conned}]

con
     v 1: deprive of by deceit; "He swindled me out of my
          inheritance"; "She defrauded the customers who trusted
          her"; "the cashier gypped me when he gave me too little
          change" [syn: {victimize}, {swindle}, {rook}, {goldbrick},
           {nobble}, {diddle}, {bunco}, {defraud}, {scam}, {mulct},
           {gyp}]
     2: commit to memory; learn by heart; "Have you memorized your
        lines for the play yet?" [syn: {memorize}, {memorise}, {learn}]
     [also: {conning}, {conned}]

Source : Free On-Line Dictionary of Computing

con
     
        [SF fandom] A science-fiction convention.  Not used of other
        sorts of conventions, such as professional meetings.  This
        term, unlike many others of SF-fan slang, is widely recognised
        even by hackers who aren't {fan}s. "We'd been corresponding on
        the net for months, then we met face-to-face at a con."
     
        [{Jargon File}]