Source : Webster's Revised Unabridged Dictionary (1913)

Piddock \Pid"dock\, n. [Etymol. uncertain.] (Zo["o]l.)
   Any species of Pholas; a pholad. See {Pholas}.

Source : WordNet®

piddock
     n : marine bivalve that bores into rock or clay or wood by means
         of saw-like shells