look up any word, like bukkake:
 
1.
knee sex is when while making out with a guy(laying down), he carefully rubs his knee against a girl's crotch to stimulate her vagina and get it ready for fingering and/or sex.
girl 1: I wasn't going to go past 2nd base with him, but he started doing something with his knee and i went crazy.

girl 2: O he did the knee sex move. yea he does that well.
by cavillea April 30, 2005
 
2.
Very careful targetted groping of the knee, as well as under, below and above the knee to achieve knee-orgasm. Preferably performed by more than 2 people (gender is not an issue with knee sex)
Who's up for some knee sex?
by Anil Vij January 20, 2004
 
3.
The lastest and greatest form of sex. It is done when two knees are pushed and rubbed together. It usually causes brusing.
Omg look at the bruises on her knee, she must of had some good knee sex last night!

You're a knee sex slut!
by Blnde14 January 20, 2009