The Collaborative International Dictionary of English v.0.48:

Tank \Tank\ (t[a^][ng]k), n. [Pg. tanque, L. stangum a pool; or
   perhaps of East Indian origin. Cf. Stank, n.]
   1. A large basin or cistern; an artificial receptacle for
      liquids.
      [1913 Webster]

   2. A pond, pool, or small lake, natural or artificial.
      [Webster 1913 Suppl.]

            We stood in the afterglow on the bank of the tank
            and saw the ducks come home.          --F.
                                                  Remington.

            The tanks are full and the grass is high. --Lawson.
      [Webster 1913 Suppl.]

   3. (Mil.) a heavily armored combat vehicle which moves on
      caterpillar treads, rather than wheels. It typically
      carries a cannon and a heavy machine, and sometimes other
      weapons. It is the main distinguishing weapon of an
      armored division.
      [PJC]

   4. a jail cell for temporarily holding prisoners, as in a
      police station.
      [PJC]

   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 (Zool.), a small nematoid worm found in the water
      tanks of India, supposed by some to be the young of the
      Guinea worm.
      [1913 Webster]

WordNet (r) 3.0 (2006):

tank iron
    n 1: plate iron that is thinner than boilerplate but thicker
         than sheet iron or stovepipe iron