Ours mal léché

Literally: A badly licked bear

Example: “Le gardien du musée est un ours mal léché qui ne dit jamais bonjour.” — The museum guard is a boor who never says hello.

Origin: The bear is a solitary animal, which accounts for the first sense — someone who shuns company — easily enough. The second takes more explaining, and rests on a belief far older than the medieval bestiaries that repeated it: already in Aristotle's time the bear cub was held to be born half-formed, and finished by its mother licking it into the shape of an animal. One licked insufficiently stayed rough. The belief outlived its refutation; a text of 1579 reports hunters in the val d'Anania cutting open a large female bear and finding the cubs inside with their limbs already distinct and formed, exactly as the writer says, against Aristotle and Pliny both. What changed was not the truth of it but the target: a supposed malformation of the body became a malformation of the upbringing, so the badly licked man is the badly raised one, and therefore coarse.