housemaids knee

from The Collaborative International Dictionary of English v.0.48

Housemaid \House"maid`\, n.
   A female servant employed to do housework, esp. to take care
   of the rooms.
   [1913 Webster]

   {Housemaid's knee} (Med.), a swelling over the knee, due to
      an enlargement of the bursa in the front of the kneepan;
      -- so called because frequently occurring in servant girls
      who work upon their knees.
      [1913 Webster]