The Collaborative International Dictionary of English v.0.48:

Fermillet \Fer"mil*let\, n. [OF., dim. of fermeil, fermail,
   clasp, prob. fr. OF. & F. fermer to make fast, fr. ferme
   fast. See Firm.]
   A buckle or clasp. [Obs.] --Donne.
   [1913 Webster]