Scratching your knee 

Euphemism relating to masturbation. Generally used when caught red handed and in desperate need of an excuse to avert suspicion.

Did you get caught in your hotel room scratching your knee again?

Scratching your knee by Fray3k May 14, 2014

Scratching Your Knee 

A term that is used as an excuse for when you’re caught jerking one out. Could also mean jerking off in general.

Guy 1: Hey man we’re you just wanking it?!?!
Guy 2: Nah, just scratching my knee.
Guy 1: Oh, alright, continue scratching your knee then.

Scratching Your Knee by TheRussianHammer May 16, 2018