NOUN (3)

1. a sharp metal spike or spur that is fastened to the leg of a gamecock;

2. a spar rising aft from a mast to support the head of a quadrilateral fore-and-aft sail;

3. an iron hook with a handle; used for landing large fish;

The Collaborative International Dictionary of English v.0.48:

Gaff \Gaff\ (g[a^]f), n. [OE. gaffe, F. gaffe an iron hook with
   which seamen pull great fishes into their ships; cf. Ir. gaf,
   gafa hook; perh. akin to G. gabel fork, Skr. gabhasti. Cf.
   Gaffle, Gable.]
   1. A barbed spear or a hook with a handle, used by fishermen
      in securing heavy fish.
      [1913 Webster]

   2. (Naut.) The spar upon which the upper edge of a
      fore-and-aft sail is extended.
      [1913 Webster]

   3. Same as Gaffle, 1. --Wright.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Gaff \Gaff\, v. t. [imp. & p. p. Gaffed; p. pr. & vb. n.
   Gaffing.]
   To strike with a gaff or barbed spear; to secure by means of
   a gaff; as, to gaff a salmon.
   [1913 Webster]

WordNet (r) 3.0 (2006):

gaff
    n 1: a sharp metal spike or spur that is fastened to the leg of
         a gamecock
    2: a spar rising aft from a mast to support the head of a
       quadrilateral fore-and-aft sail
    3: an iron hook with a handle; used for landing large fish