In ancient England, People could not have sex without consent from the King.
When people wanted to have a child, they had to solicit a permision to the monarchy, in turn they would supply
a plaque to hang on their door when they had sexual relations. The Plaque read
"Fornication Under Consent of the King" (F.U.C.K.).

This is the origin of the word.
(F.U.C.K.)