housemaid's knee

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A swelling of the bursa in front of the patella just beneath the skin, caused by trauma, such as that brought about by excessive kneeling.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Swelling of the bursa before the kneecap.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun swelling of the bursa in the knee (due to trauma or excessive kneeling)