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1.
the back of one's knee

"Those sit ups made my kneenuses really sweaty!"
by cheese-eater February 27, 2009
 
2.
The skin on your knees. It can be stretched out, and, like a weenus, you can compare the size of it to others.
Hey check the size of my kneenus
My kneenus is bigger than yours
Hey look at timmy, I bet he has a small kneenus
by Scott F. December 05, 2007