Source : Webster's Revised Unabridged Dictionary (1913)

Harpoon \Har*poon"\, n. [F. harpon, LL. harpo, perh. of Ger.
   origin, fr. the harp; cf. F. harper to take and grasp
   strongly, harpe a dog's claw, harpin boathook (the sense of
   hook coming from the shape of the harp); but cf. also Gr. ?
   the kite, sickle, and E. harpy. Cf. {Harp}.]
   A spear or javelin used to strike and kill large fish, as
   whales; a harping iron. It consists of a long shank, with a
   broad, fiat, triangular head, sharpened at both edges, and is
   thrown by hand, or discharged from a gun.

   {Harpoon fork}, a kind of hayfork, consisting of bar with
      hinged barbs at one end a loop for a rope at the other
      end, used for lifting hay from the load by horse power.

   {Harpoon gun}, a gun used in the whale fishery for shooting
      the harpoon into a whale.

Source : WordNet®

harpoon gun
     n : a cannon or cannon-like gun that fires harpoons