The Collaborative International Dictionary of English v.0.48:

Stifle \Sti"fle\, n. [From Stiff.] (Far.)
   The joint next above the hock, and near the flank, in the
   hind leg of the horse and allied animals; the joint
   corresponding to the knee in man; -- called also stifle
   joint. See Illust. under Horse.
   [1913 Webster]

   Stifle bone, a small bone at the stifle joint; the patella,
      or kneepan.
      [1913 Webster]