The Collaborative International Dictionary of English v.0.48:

Con- \Con-\
   A prefix, fr. L. cum, signifying with, together, etc. See
   Com-.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

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.]
      [1913 Webster]

            Of muses, Hobbinol, I con no skill.   --Spenser.
      [1913 Webster]

            They say they con to heaven the highway. --Spenser.
      [1913 Webster]

   2. To study in order to know; to peruse; to learn; to commit
      to memory; to regard studiously.
      [1913 Webster]

            Fixedly did look
            Upon the muddy waters which he conned
            As if he had been reading in a book.  --Wordsworth.
      [1913 Webster]

            I did not come into Parliament to con my lesson.
                                                  --Burke.
      [1913 Webster]

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

   To con thanks, to thank; to acknowledge obligation. [Obs.]
      --Shak.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

con
    adv 1: in opposition to a proposition, opinion, etc.; "much was
           written pro and con" [ant: pro]
    n 1: an argument opposed to a proposal [ant: pro]
    2: a person serving a sentence in a jail or prison [syn:
       convict, con, inmate, yard bird, yardbird]
    3: a swindle in which you cheat at gambling or persuade a person
       to buy worthless property [syn: bunco, bunco game,
       bunko, bunko game, con, confidence trick, confidence
       game, con game, gyp, hustle, sting, flimflam]
    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, gip, hornswoggle, short-change, con]
    2: commit to memory; learn by heart; "Have you memorized your
       lines for the play yet?" [syn: memorize, memorise, con,
       learn]

The Jargon File (version 4.4.7, 29 Dec 2003):

con
 n.

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