Un soleil de plomb

Literally: A sun of lead

Example: “Les vendangeurs ont travaillé sous un soleil de plomb.” — The grape pickers worked under a blazing, oppressive sun.

Origin: Lead supplies weight, not temperature, so the sun here presses down rather than burns — which is what the heat of a real summer actually feels like. De plomb attached itself to the sun in 1835 and to sleep in 1842, and French reaches for the same metal in un ciel de plomb. It goes lower, too: a cul de plomb is an office worker, permanently seated.