The Collaborative International Dictionary of English v.0.48:

Knack-kneed \Knack"-kneed`\, a.
   See Knock-kneed.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

knock-kneed \knock"-kneed`\, a.
   Having the legs bent inward so that the knees touch in
   walking. [Written also knack-kneed.]