Clergyman's knee is a swelling below the knee, properly called infrapatellar bursitis - inflammation in the infrapatellar bursa.

clergyman
Long periods pressing on the infrapatellar bursa may cause it to become inflamed.
Synonyms: 
Infrapatellar bursitis
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Dr Sheila StroverĀ (Editor)
BSc (Hons), MB BCh, MBA

See biography...