Mettre les pieds dans le plat

Literally: To put one's feet in the dish

Example: “Il a mis les pieds dans le plat en parlant de la fête surprise devant elle.” — He put his foot in it by mentioning the surprise party in front of her.

English equivalent: To put one's foot in it

Origin: From the start of the 19th century, and better explained than the obvious reading suggests. The obvious version — literally planting your feet in the serving dish at dinner, a blunder nobody could miss — is almost too easy, and Pierre Guiraud's account in Les locutions françaises offers something less tidy but better supported: a pun between Franco-Provençal words for a ford and for wading or paddling through water, and plat in the older sense of a shallow, flat stretch of water. Blundering by wading straight through a shallow ford is a clumsier, muddier image than stepping in a dinner platter, and stirring up mud in shallow water is exactly what a tactless remark does to a conversation that had been running smoothly.