Avoir la tête dans les nuages

Literally: To have one's head in the clouds

Example: “Elle a encore oublié ses clés, elle a toujours la tête dans les nuages.” — She forgot her keys again; she always has her head in the clouds.

English equivalent: To have one's head in the clouds

Origin: Genuinely hard to pin down. Clouds and the sky generally have carried associations with elevated or divine states of mind across a great many cultures, and medieval writers already reached for celestial imagery to describe minds that had wandered off from the everyday — but tracing the specific French wording back to a first attestation is not something any source manages convincingly. Traces of the image turn up as early as the 16th century, and the phrase in something like its current form was in wide use by the 18th, which is as precise as the record gets: a head placed somewhere its owner's feet plainly are not, standing in for a mind that has drifted the same distance from what's actually in front of it.