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
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.