Source : Webster's Revised Unabridged Dictionary (1913)

Joker \Jok"er\, n.
   1. One who makes jokes or jests.

   2. (Card Playing) See {Rest bower}, under 2d {Bower}.

Bower \Bow"er\ (bou"[~e]r), n. [G. bauer a peasant. So called
   from the figure sometimes used for the knave in cards. See
   {Boor}.]
   One of the two highest cards in the pack commonly used in the
   game of euchre.

   {Right bower}, the knave of the trump suit, the highest card
      (except the ``Joker'') in the game.

   {Left bower}, the knave of the other suit of the same color
      as the trump, being the next to the right bower in value.
      

   {Best bower} or {Joker}, in some forms of euchre and some
      other games, an extra card sometimes added to the pack,
      which takes precedence of all others as the highest card.

Source : WordNet®

joker
     n 1: a person who enjoys telling or playing jokes [syn: {jokester}]
     2: a person who does something thoughtless or annoying; "some
        joker is blocking the driveway" [syn: {turkey}]