There's confusion when a word means one thing to one set of people, and the opposite to the other set of people. When someone says they're "nonplussed", what e

There's confusion when a word means one thing to one set of people, and the opposite to the other set of people. When someone says they're "nonplussed", what exactly are they saying? The choices are to either agree more broadly to a single meaning ("change the meaning" to half of the people), or find another less ambiguous word.