Vocabulaire de la salle de bain en français

How do you pronounce “shampooing” in French?

French borrowed the English word shampooing and kept the spelling, but not the sound. It comes out roughly as shahn-pwan — /ʃɑ̃pwɛ̃/, with nasal vowels and no trace of “sham-poo” left in it — one of the sharpest reminders that a familiar-looking word on a French label tells you nothing about how to say it. The spelling is drifting too: shampoing with one o is now the more common form, though bottles still often print shampooing.

The words French built on top of it went the other way and respell the sound outright: shampouiner (to shampoo), un shampouineur / une shampouineuse (the person, and the carpet machine). Those -oui- spellings are the pronunciation written down.