SWP's back said:
Genuine question, not taking the piss.
Should it be 'for fucks sake' or 'for fuck's sake'?
I believe the original phrase, 'for Pete's sake' comes from medieval times when people were more religious and less inclined to swear. When they made an error or hurt themselves they would, rather than saying 'for fuck's sake' or 'for God's sake', would say 'for St Peter's sake' which eventually became 'for Pete's sake'.
So as we've just replaced the word Pete with the word fuck, surely the apostrophe should remain?