Ça me fait une belle jambe

Literally: That makes me a fine leg

Example: “Tu as gagné un bon de réduction pour un parachute ? Ça me fait une belle jambe.” — You won a discount voucher for a parachute? A fat lot of good that does me.

English equivalent: A fat lot of good that does me

Origin: The leg in question is a man's, not a woman's — the phrase mocks male vanity rather than admiring female charm. From the 15th century especially, men wore chausses that showed the leg off: the haut-de-chausses to about the knee and the bas-de-chausses, ancestor of the stocking, clinging close enough to leave the calf's shape barely to the imagination. By the 17th century a well-turned leg mattered socially, and the men who paraded theirs gave French faire la belle jambe. Furetière records the ironic turn by the end of that century — cela ne me rendra pas la jambe mieux faite, that will not make my leg any better shaped, said to someone offering something useless. Drop the negation and you have the modern antiphrasis; ça me fait bien la jambe appears in the 19th century before the current wording won out.