Con comme une buse

Literally: Stupid as a buzzard

Example: “Il a laissé ses clés sur le contact, portes ouvertes ; il est con comme une buse.” — He left his keys in the ignition with the doors open; he's dumb as a box of rocks.

English equivalent: Dumb as a box of rocks

Origin: Con here is the common vulgar-register adjective for "stupid," not its older anatomical sense. Buse is the buzzard, a bird of prey that medieval and early-modern falconers ranked at the bottom of the hierarchy of hunting birds: unlike falcons and hawks, it was considered too sluggish and unteachable to be trained for the hunt. That reputation for uselessness gave buse its figurative sense of "fool" or "ignoramus" by at least the 16th century, and the phrase simply pairs that old sense with the blunt modern intensifier.