Con comme la lune

Literally: As stupid as the moon

Example: “Il a mis de l'essence dans une voiture diesel ; il est con comme la lune.” — He put gasoline in a diesel car; he's dumb as a box of rocks.

English equivalent: Dumb as a box of rocks

Origin: Recent as these things go — attested only from the start of the 20th century — but built on a much older association. Since the 17th century the moon has stood for absent-mindedness or mild derangement: être dans la lune (to be in the moon) is to have your thoughts elsewhere, and avoir un quartier de lune dans la tête (to have a quarter-moon in the head) marks someone odd. Two readings compete for the stupidity. One takes the full moon as a face — round, blank, entirely without expression, which is the look being described. The other is straight slang, where lune means the backside on a simple resemblance of shape: a part of the anatomy poorly thought of, and traditionally set against the head.