It’s a corruption of the construction ‘For God’s sake’ so in standard English I’d favour ’For fuck’s sake’ but as language evolves, in colloquial English the simple contraction ’fucksake’ has become common parlance. In 20+ years in newsrooms, I’d say barely an hour goes by without hearing ‘fucksake’
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Bravo.
OR for the sake of many fucks = for fucks' sake.
Depends how many fucks are involved 😊