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

\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.