Definition of knee:
in humans, the joint
in the middle part of the leg
; the joint, or region of the joint, between the thigh and leg
"jessica was wearing shorts, so she skinned her exposed knees when she fell."
"1605: I could as well be brought / To knee his throne and, squire-like, pension beg / To keep base life afoot. -- William Shakespeare, King Lear II.ii"