Faut pas pousser mémé dans les orties

Literally: You mustn't push granny into the nettles

Example: “Je veux bien te prêter ma voiture, mais la garder un mois, faut pas pousser mémé dans les orties.” — I'm happy to lend you my car, but keeping it a month? Don't push your luck.

English equivalent: Don't push your luck

Origin: The base of it is simply faut pas pousser — don't overdo it — whose own origin is unknown and whose appearance is put in the first half of the 20th century. The metaphor underneath is unambiguous: push too hard and you damage whatever you are pushing. Mémé and the nettles are pure reinforcement, and the comedy of the image is why this version is heard far more often than faut pas pousser le bouchon trop loin (don't push the cork too far), which means exactly the same thing.