:In ancient England a person could not have sex unless you had consent of the King (unless you were in the Royal Family). When anyone wanted to have a baby, they got consent of the King, the King gave them a placard that they hung on their door while they were having sex. The placard had F.*.*.*. (Fornication Under Consent of the King) on it. Now you know where that came from.
make of it what you will
It's an urban myth. If everyone who wanted to fornicate had to ask permission of the King, the King wouldn't have enough time to eat or sleep.