Source : Webster's Revised Unabridged Dictionary (1913)

Geoduck \Ge"o*duck\, n. [American Indian name.] (Zo["o]l.)
   A gigantic clam ({Glycimeris generosa}) of the Pacific coast
   of North America, highly valued as an article of food.

Source : WordNet®

geoduck
     n : a large edible clam found burrowing deeply in sandy mud
         along the Pacific coast of North America; weighs up to
         six pounds; has siphons that can extend to several feet
         and cannot be withdrawn into the shell