The Collaborative International Dictionary of English v.0.48:

Harpoon \Har*poon"\ (h[aum]r*p[=oo]n"), 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. "a`rph 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, flat, triangular head, sharpened at both edges, and is
   thrown by hand, or discharged from a gun.
   [1913 Webster]

   Harpoon fork, a kind of hayfork, consisting of a bar with
      hinged barbs at one end and 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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

harpoon gun
    n 1: a cannon or similar gun that fires harpoons