Source : Webster's Revised Unabridged Dictionary (1913)

Thoroughpin \Thor"ough*pin`\, n. (Far.)
   A disease of the hock (sometimes of the knee) of a horse,
   caused by inflammation of the synovial membrane and a
   consequent excessive secretion of the synovial fluid; --
   probably so called because there is usually an oval swelling
   on each side of the leg, appearing somewhat as if a pin had
   been thrust through.