a girl that oral sex to a guy
yo, she got dirty knees!
When they came out of the bathroom her hair was mussed; she might as well have had dirty knees.
To Suck someones penis, while on your knee
yo, she be getting her knees dirty.
Yo she got dirty knees.
when a girl gives a boy head in a porta-potty, which results in her knees being covered in fecal material or piss. Once the man blows a load, and they clean up, the girl needs to use soap and water before her knees are to the original skin color.
Samantha has some dirty knees after the porta-potty incident.