The Collaborative International Dictionary of English v.0.48:

Bunyon \Bun"yon\, Bunion \Bun"ion\, n. [Cf. Prov. E. bunny a
   small swelling, fr. OF. bugne, It. bugna, bugnone. See
   Bun.] (Med.)
   An enlargement and inflammation of a small membranous sac
   (one of the burs[ae] muscos[ae]), usually occurring on the
   first joint of the great toe.
   [1913 Webster]