Source : Webster's Revised Unabridged Dictionary (1913)

Borer \Bor"er\, n.
   1. One that bores; an instrument for boring.

   2. (Zo["o]l.)
      (a) A marine, bivalve mollusk, of the genus {Teredo} and
          allies, which burrows in wood. See {Teredo}.
      (b) Any bivalve mollusk ({Saxicava}, {Lithodomus}, etc.)
          which bores into limestone and similar substances.
      (c) One of the larv[ae] of many species of insects, which
          penetrate trees, as the apple, peach, pine, etc. See
          {Apple borer}, under {Apple}.
      (d) The hagfish ({Myxine}).

Source : WordNet®

borer
     n 1: a drill for penetrating rock [syn: {bore bit}, {rock drill},
           {stone drill}]
     2: any of various insects or larvae or mollusks that bore into
        wood [syn: {woodborer}]