Nonplussed means so surprised that you have no idea what to do next. Stopped in your tracks.
The word is currently splitting in two, and it is worth knowing why. The original meaning is bewildered, from a Latin phrase meaning no more can be said. In American English it has come to mean the opposite — unbothered, unfazed — probably because the non- looks like a negative. Careful writers avoid it altogether now, since half the readers will take it one way and half the other.
1adjectivenonplussedSo surprised or confused that you do not know how to react.
“He was completely nonplussed by the question.”
Nonplussed is Latin for no further. Nōn plūs meant no more, and a nonplus was the point at which nothing else can be said or done, so the person is stopped rather than merely surprised.
Proto-Indo-European
*pleh₁- — to fill
Latin
nōn plūs — no further, no more
English, today
nonplussed
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Etymology from Wiktionary, CC BY-SA