Nonplussed most safely means confused, puzzled, or unsure how to react. Cambridge defines it as “surprised, confused, and not certain how to react,” and Oxford gives essentially the same meaning. Merriam-Webster lists that as the primary sense too.

There is a complication: Merriam-Webster also notes that in modern American usage, nonplussed is sometimes used to mean unimpressed or unfazed, but it says that sense is still widely regarded as an error. So for clean, reliable English, the safest meaning is still bewildered.

✅ She looked completely nonplussed when the presentation started in the wrong language.

Here, nonplussed means confused and unsure what to do. That matches Cambridge and Oxford directly.

✅ He seemed nonplussed by the question and took a moment before answering.

Again, the meaning is puzzled or caught off guard.

⚠️ She was nonplussed by the criticism.

Some readers will take this to mean confused. Others may read it as unfazed. Merriam-Webster explains that this split is exactly why the word causes so much confusion.

✅ She was unfazed by the criticism.
✅ She was unimpressed by the criticism.

If that is your intended meaning, these are clearer than nonplussed. Merriam-Webster specifically notes the newer “unimpressed” sense, but also notes that many people still reject it.

The most common mistake is using nonplussed to mean not bothered or not impressed and assuming everyone will understand it that way. Many people will not. Cambridge and Oxford stick to the confused / puzzled meaning, while Merriam-Webster documents the newer opposite-ish sense but flags it as controversial.

So this is one of those words where being “fancy” can accidentally make you less clear.

Use this rule:

A simple memory trick:

nonplussed = no plus, no next move

That fits the older core sense Merriam-Webster traces: being so perplexed you do not know what to say, think, or do.