Être à l'ouest

Literally: To be in the west

Example: “Depuis qu'il est rentré de voyage, il est complètement à l'ouest.” — Since he got back from his trip, he's been completely out to lunch.

English equivalent: Out to lunch

Origin: Recent — the end of the 20th century — and most likely an import. English to go west meant to die or be killed around the First World War, and in thieves' slang before that, to be hanged. Crossing the Channel it lost its force, death softening into a general daze. Two other accounts circulate. One puts it in the theater at the start of the 20th century, when the workshops and stages sat mostly in eastern Paris while the actors lived in the west, so a performer went home westward at the end of the night with nothing left in him. The other is unlikely but enjoyable: Professor Tournesol in Tintin is permanently à côté de la plaque, and in Le Trésor de Rackham le Rouge he insists his pendulum is telling him to go further and further west.