The space in between your knees. This is not a real knee, but another interesting body part.
Dude you wouldn't believe she sucked my middle knee.
The part between the left and right knees, also referred to as the: dick, penis, cock, big Johnson, etc.
He should've carried her up the stairs to her room and tied her to the bed and touched that middle knee all night long.
The inbetween knee, if you know what I mean
Dude, she touched my middle knee!