Avoir un pet de travers

Literally: To have a fart sideways

Example: “Le chef a un pet de travers ce matin, ne le dérange pas.” — The boss is a bit under the weather this morning; don't bother him.

English equivalent: Under the weather

Origin: Pet arrives in the 12th century from Latin peditum, off the verb pedere, and means exactly what it means. By the 13th it had also taken the figurative sense of a thing of no value at all — for reasons nobody explains — which is what stands behind ne pas valoir un pet, later extended with an animal, most often the rabbit. The expression itself needs no scholarship: anyone who has had trapped wind knows how thoroughly it can sour an afternoon, and that is enough to have produced, in the first half of the 20th century, a way of accounting for someone who is either mildly ill or simply impossible to be around.