désarçonner

Literally "to unseat" a rider from a horse — from arçon, the saddle bow — but used just as often today in its figurative, informal sense: to throw someone off balance mentally, catch them off guard in an argument or question (une question qui désarçonne).

• Le cheval a désarçonné son cavalier d'un bond. — The horse threw its rider with a single buck.
• Sa question m'a complètement désarçonné. — His question completely threw me off.
• Elle n'est jamais désarçonnée, même face à un public hostile. — She's never thrown off, even facing a hostile audience.

Learners often only know the literal horse-riding sense and miss that the figurative "catching someone off guard" meaning is now by far the more common everyday use.