Être un ours

Literally: To be a bear

Example: “Mon voisin est un ours, il ne dit jamais bonjour.” — My neighbor is a real grouch; he never even says hello.

Origin: Rests on the animal's ordinary behavior rather than any old legend: a bear lives alone by nature, tolerating company only briefly and for its own reasons, so calling an unsociable, brusque person a bear needs no folklore to explain it — the comparison is just accurate zoology applied to a person who avoids others the same way. This is the plainer of two French bear insults; the harsher ours mal léché (badly licked bear) carries its own separate story about a supposed upbringing gone wrong, which is not what this simpler phrase is claiming.