En rester comme deux ronds de flan

Literally: To be left like two rounds of custard

Example: “Quand il a annoncé qu'il partait vivre en Islande, j'en suis resté comme deux ronds de flan.” — When he said he was moving to Iceland, you could have knocked me down with a feather.

English equivalent: You could have knocked me down with a feather

Origin: Two circles of flan side by side do make a fair picture of eyes wide with astonishment — but Alain Rey and Sophie Chantreau, about the only authorities to take the phrase on, are not convinced the custard has anything to do with it. Esnault cites it in 1901, so it dates from the end of the 19th century, and every explanation has a hole in it. A flaon or flan was, in the 14th century, the metal blank struck by a die to become a coin, which would make this a pun on frapper — to strike a coin, to be struck with stupor — except that the word had dropped out of use by the 19th century. In printing, a flan is the coated card that takes the impression of a composition, which explains nothing about why round or why two. And a last, frankly strained suggestion has it as flanc with the c worn off, deux ronds de flanc being the buttocks, so that the astonished party is left on his backside.