Être à côté de la plaque

Literally: To be next to the plate

Example: “On lui a demandé le prix et il a parlé de la couleur : il est à côté de la plaque.” — We asked him the price and he talked about the color; he's wide of the mark.

English equivalent: Wide of the mark

Origin: Unsettled, though one candidate is much likelier than the other. Discount the cooking plate, the dental plate and the manhole cover, and what remains first is shooting: the plaque is the target panel — a straw boss in archery, a round metal plate in some pistol events — so a shot that misses lands beside it. The explanation is unglamorous, which is exactly why it is probably right: it describes something a great many people did, and phrases spread from what people do. The rival account comes from the railways, where a roundhouse turntable sits over a pit, and a driver who moved his locomotive before the pivoting section was properly lined up put it beside the plate and into the hole. Vivid, but confined to a small trade.