Source : Webster's Revised Unabridged Dictionary (1913)

Filefish \File"fish`\, n. (Zo["o]l.)
   Any plectognath fish of the genera {Monacanthus}, {Alutera},
   {balistes}, and allied genera; -- so called on account of the
   roughly granulated skin, which is sometimes used in place of
   sandpaper.

Source : WordNet®

filefish
     n : narrow flattened warm-water fishes with leathery skin and a
         long file-like dorsal spine
     [also: {filefishes} (pl)]