C'est comme pisser dans un violon

Literally: It's like peeing in a violin

Example: “Tu peux lui expliquer cent fois, c'est comme pisser dans un violon : il n'écoute jamais.” — You can explain it to him a hundred times, it's completely pointless: he never listens.

Origin: Late 19th century, and unexplained in any documented way — every source agrees on that much. Alain Rey's guess is that pisser simply replaced an earlier verb like souffler or siffler (to blow, to whistle) for comic shock value, so the original image was a musician blowing uselessly into an instrument that needs bowing, not breath. A second theory reaches for a small river in Aveyron called the Vioulou, whose name is also the Occitan word for violin, and supposes the phrase started as Occitan es coma si pissavas dins un vioulou before being carried into French with the sense intact and the logic lost in translation. Pisser itself is far older and was not always coarse: it comes from a form of Latin meaning simply to urinate, and stayed the ordinary word for the act — uriner being reserved for doctors — until the 19th century decided it was vulgar.