The Collaborative International Dictionary of English v.0.48:

Man-of-war \Man`-of-war"\, n; pl. Men-of-war.
   1. A government vessel employed for the purposes of war, esp.
      one of large size; a ship of war. [WordNet sense 1]

   Syn: ship of the line.
        [1913 Webster]

   2. The Portuguese man-of-war.

   Syn: Syn. --, jellyfish.
        [WordNet 1.5]

   Man-of-war hawk (Zool.), the frigate bird.

   Man-of-war's man, a sailor serving in a ship of war.

   Portuguese man-of-war (Zool.), any species of the genus
      Physalia; it is a hydrozoan having both medusa and polyp
      stages present in a single colony. It floats on the
      surface of the sea by a buoyant bladderlike structure,
      from which dangle multiple long tentacles with stinging
      cells. Its can cause severe rashes when it comes in
      contact with humans swimming in the area. See Physalia.
      [1913 Webster]

WordNet (r) 3.0 (2006):

man-of-war
    n 1: a warship intended for combat [syn: man-of-war, ship of
         the line]
    2: large siphonophore having a bladderlike float and stinging
       tentacles [syn: Portuguese man-of-war, man-of-war,
       jellyfish]