nonplussed (or nonplused): Flummoxed to the point of inaction. "At the debate, Eric's references to Freudian physiology, ichthyological anthropology, and mythological proctology left Joan nonplussed; she spoke not a word during the two minutes alloted for her rebuttal."
Nonplussed is one of those words that throws people off because it uses a common prefix — which points us toward understanding — at the head of an uncommon word. We hear of people being nonplussed, but rarely do we hear of people being plussed. Along the same lines, you've probably been nonchalant at times, but have you ever been called chalant? I've been incorrigible before, but no one has ever mentioned those times when I've been corrigible. You know indubitable facts, but have you ever wondered about something dubitable?
Nonchalant, incorrigible, and indubitable are used often enough in conversation and in print that, in general, we understand what they mean without having to analyze the different parts of the word. We understand them as stand-alone words, not as words altered by prefixes. Nonplussed isn't so well-used, and is misused often enough to muddy the waters even more.
(An aside: I wonder which is more often misused: nonplussed or comprised?)
According to Webster's New World College Dictionary, nonplussed comes from the Latin non- "not" and plus "more." So you might think of nonplussed as meaning "not being able to do anymore." The derivation doesn't really explain how the idea of perplexity (or flummoxity) entered into the meaning, though.
What nonplussed ought to mean: Not added together, as in this sentence, which might appear on an epoxy dispenser: "The epoxide and polyamide should remain nonplussed until right before you're ready to start binding items together permanently."