The Collaborative International Dictionary of English v.0.48:

Jess \Jess\, n.; pl. Jesses. [OF. gies, giez, prop. pl. of
   giet, get, jet, F. jet, a throwing, jess. See Jet a
   shooting forth.] (falconry)
   A short strap of leather or silk secured round the leg of a
   hawk, to which the leash or line, wrapped round the
   falconer's hand, was attached when used. See Illust. of
   Falcon.
   [1913 Webster]

         Like a hawk, which feeling freed
         From bells and jesses which did let her flight.
                                                  --Spenser.
   [1913 Webster]