Quick question. Is it "for fuck sake" or "for fucks sake?

It's for a work email, so it has to sound professional.

Proof reader #1. Neither: It's for fuck's sake. The sake is on behalf of the fuck, so you need a possessive.

Proof reader #2. It should have an apostrophe to show possessive. The REAL question is, should it be "for fuck's sake" or "for fucks' sake"? Is if for the sake of ALL the fucks or just this fuck in particular?

Quick question. Is it "for fuck sake" or "for fucks sake?
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