The Collaborative International Dictionary of English v.0.48:

Overboard \O"ver*board`\, adv.
   Over the side of a ship; hence, from on board of a ship, into
   the water; as, to fall overboard.
   [1913 Webster]

   To throw overboard, to discard; to abandon, as a dependent
      or friend.

   To go overboard, to go to an extreme; to overdo; as, he
      went overboard at the buffet and got an upset stomach.
      [1913 Webster +PJC]