From French 'bouder' (to sulk) + '-oir' (place for) — literally 'a room for sulking in.'
From French 'boudoir,' literally 'a place to sulk in' — from 'bouder' (to sulk, pout). A boudoir was the room a woman withdrew to when she wanted to be alone with her bad mood. Key roots: bouder (French: "to sulk, pout").
A 'boudoir' is a sulking room. French 'bouder' means to pout, and the '-oir' suffix means 'a place for' (like 'parloir' is a place for talking, 'abattoir' is a place for slaughter). So a boudoir is architecturally designated for having a bad mood. The word's later association with romance and luxury photography is a complete departure from its grumpy origins.