Source : Webster's Revised Unabridged Dictionary (1913)

Tank \Tank\, n. [Pg. tanque, L. stangum a pool; or perhaps of
   East Indian origin. Cf. {Stank}, n.]
   A large basin or cistern; an artificial receptacle for
   liquids.

   {Tank engine}, a locomotive which carries the water and fuel
      it requires, thus dispensing with a tender.

   {Tank iron}, plate iron thinner than boiler plate, and
      thicker than sheet iron or stovepipe iron.

   {Tank worm} (Zo["o]l.), a small nematoid worm found in the
      water tanks of India, supposed by some to be the young of
      the Guinea worm.

Source : WordNet®

tank iron
     n : plate iron that is thinner than boilerplate but thicker than
         sheet iron or stovepipe iron