Avoir une veine de cocu

Literally: To have a cuckold's luck

Example: “Il a trouvé un billet de cent euros dans la rue ; il a une veine de cocu.” — He found a hundred-euro bill in the street; he has the luck of the devil.

English equivalent: To have the luck of the devil

Origin: First, why veine means luck at all. It comes from Latin vena, which had a figurative sense of poetic or artistic inspiration — still visible in être en veine, to be on form. From there French took two further senses: a good disposition, in the 12th century, since a poet with inspiration is well disposed to write; and then, in the middle of the 14th, plain luck, which is harder to account for but gave us n'avoir aucune veine and its opposite. The cuckold and the hanged man are both 19th century. The cuckold's claim rests on a folk principle of compensation — a man dealt that particular misfortune cannot decently be pursued by every other one as well, so he is owed a run of luck by way of settlement, and the phrase is used above all at the card table, where heureux au jeu, malheureux en amour says the same thing from the other end. The hanged man had less obvious grounds, having just been hanged. But his remains were held to be powerful: the fat of a hanged man was a cure-all, his bones warded off curses, and the rope that killed him was infallible at cards. He was a source of limitless good fortune, strictly for other people.