Source : Webster's Revised Unabridged Dictionary (1913)

Fid \Fid\, n. [Prov. E. fid a small, thick lump.]
   1. (Naut.) A square bar of wood or iron, used to support the
      topmast, being passed through a hole or mortise at its
      heel, and resting on the trestle trees.

   2. A wooden or metal bar or pin, used to support or steady
      anything.

   3. A pin of hard wood, tapering to a point, used to open the
      strands of a rope in splicing.

   Note: There are hand fids and standing fids (which are larger
         than the others, and stand upon a flat base). An iron
         implement for this purpose is called a marline spike.

   4. (Mil.) A block of wood used in mounting and dismounting
      heavy guns.