Okay, I'm not sure if this is fact but I have heard this: In ancient England a person could not have sex unless they had the King's consent (unless you were royalty). If anyone wanted to have a baby, the couple would again have to get consent from the King. Once that happened he would then give them a placard that they would hang on their door while they were having sex. The placard would have F*** (fornication under consent of the king) imprinted on it, coining our fave four-letter word.