The Collaborative International Dictionary of English v.0.48:

Boatswain \Boat"swain\, n. [Boat + swain.]
   1. (Naut.) An officer who has charge of the boats, sails,
      rigging, colors, anchors, cables, cordage, etc., of a
      ship, and who also summons the crew, and performs other
      duties.
      [1913 Webster]

   2. (Zool.)
      (a) The jager gull.
      (b) The tropic bird.
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   Boatswain's mate, an assistant of the boatswain. --Totten.
      [1913 Webster]