Actually, I know the answer to that one. In the years following the Norman invasion of England, there were two languages in use; Norman french and Anglo-Saxon. For a while there were two words for everything; the snooty upperclass Norman french word and the conquered lowerclass Anglo-Saxon word. F**k was Anglo-Saxon which made it a pig language word that "nice" people didn't say. ("Eat" and "dine" are examples that are still with us from that time.)