Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

Source : WordNet®

overboard
     adv 1: to extremes; "he went overboard to please his in-laws"
     2: from on board a vessel into the water; "they dropped their
        garbage overboard"