Source : Webster's Revised Unabridged Dictionary (1913)

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

   {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.

Source : WordNet®

housemaid's knee
     n : swelling of the bursa in the knee (due to trauma or
         excessive kneeling)