The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

   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.
      [1913 Webster]