from
The Collaborative International Dictionary of English v.0.48
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.
[1913 Webster]
2. A concussion, or blow, made by water in striking, as
against the sides of a pipe or vessel containing it.
[1913 Webster]
3. A metal hammer used when heated, as by dipping in hot
water, to blister the skin, as for counterritation.
[Webster 1913 Suppl.]