The Collaborative International Dictionary of English v.0.48:

knock-knee \knock"-knee`\, n. (Med.)
   A condition in which the knees are bent in so as to touch
   each other in walking; inknee.

   Syn: knock-knees, genu valgum.
        [1913 Webster]

WordNet (r) 3.0 (2006):

knock-knee
    n 1: an inward slant of the thigh [syn: knock-knee, genu
         valgum, tibia valga]