Nonplussed means so surprised, confused, or puzzled that you don't know what to say or do next. When you're nonplussed, you're caught completely off guard, like when a teacher announces a pop quiz on material you’ve never studied, or when someone gives you an answer that makes absolutely no sense.

If your friend starts speaking to you in a language you've never heard before, you'd be nonplussed. If you open your lunchbox to find someone has replaced your sandwich with a rubber chicken, you'd probably stand there nonplussed for a moment.

Being nonplussed is temporary: it's that frozen moment of “wait, what?” before you figure out how to respond. A student might be nonplussed by a challenging riddle, standing silently while their brain tries to catch up. The word captures that specific feeling of being stumped, baffled, and at a loss all at once.

Note: Some people mistakenly use nonplussed to mean calm or unbothered, which is actually the opposite of its traditional meaning. If you see it used this way, check the context carefully.