Source : Webster's Revised Unabridged Dictionary (1913)

Water hammer \Wa"ter ham"mer\ (Physics)
   1. A vessel partly filled with water, exhausted of air, and
      hermetically sealed. When reversed or shaken, the water
      being unimpeded by air, strikes the sides in solid mass
      with a sound like that of a hammer.

   2. A concussion, or blow, made by water in striking, as
      against the sides of a pipe or vessel containing it.

Water hammer \Water hammer\
   A metal hammer used when heated, as by dipping in hot water,
   to blister the skin, as for counterritation.

Source : WordNet®

water hammer
     n : the banging sound of steam in pipes