I don't know if it's true or not, but I once heard it was shorthand for Fornication Under the Crown King - and represents the idea that - just as the idea that in most places, people dont actually OWN their land, they are simply holding a deed to title - same goes for sex and marriage. As in - you can get married and have sex under the King's (state's) permission but at any time the King (state) could come and seize your land, and your wife. Not a right, but a privilege...
Interesting thought, true or not.