A knee knocker is a specific act of casual sex, usually performed in an alley beside or behind a pub/bar/tavern.
The act is performed standing up, with the woman secured against a wall with a combination of the mans upper body and holding her buttocks. The womans legs, usually wrapped around his waist.
This unusual position, whilst having sex results in the knees of the male regularly knocking together (often with some force), hence the term.
Also refer to the term 'knee trembler'.
Grandfather: "Arrgh, I met a girl when I was serving in France, called Michelle. We had a knee knocker behind a cafe in Rennes."