spavin (n.)

disease of the hock joint of a horse, early 15c., from Middle French espavain (Modern French épavin, cognate with Italian spavenio, Spanish esparavan); in most sources said to be perhaps from Frankish *sparwan "sparrow" (see sparrow), on the supposition that a horse affected with spavin moved with a walk that reminded people of the bird's awkward gait. This seems a stretcher, and Century Dictionary admits it rests on mere resemblance of form. Related: Spavined (adj.).