Boudoir comes from the French verb bouder, meaning to pout or to sulk. In its earliest sense, a boudoir was a small private room where a woman could retire for privacy, quiet, or a little displeasure. That is why the word has been glossed as meaning a place to pout in.
Over time, the meaning shifted. In English, boudoir often came to refer to an elegant private sitting room, dressing room, or bedroom area rather than a place specifically linked with pouting.
- Earlier sense: a private retreat room
- Later sense: a stylish bedroom or dressing space
- Modern extension: boudoir photography, named for that intimate indoor setting
So the interesting part is not that people literally built rooms for pouting alone, but that the word grew out of the idea of withdrawing in a mood. The modern glamorous feel of boudoir is much newer than its older, more emotional origin.