Traditionally, so surprised or confused that one is at a loss as to how to respond.
Traditionally, so surprised or confused that one is at a loss as to how to respond; bewildered, perplexed, confounded.
In its established sense, nonplussed sits close to bewildered and confused but adds a flavor of being stopped in one's tracks, mentally stalled, unsure what to say or do next; it is the cognitive seizure that often follows surprise. It overlaps with the wonder of surprise but lacks delight, and unlike sheer confusion it implies a temporary inability to react at all. The famous complication is that a newer chiefly North American sense reverses this entirely, meaning "unbothered" or "unfazed," so the same sentence can read as either flustered or coolly indifferent depending on the reader. Because of this, careful writers often avoid the word or pin its meaning down with context; it carries no resentment, contempt, or anger like sullen or petulant, only the suspended, off-balance feeling near bewilderment.
so surprised or confused that one is unsure how to react; bewildered, at a loss
the established and original sense, dating to c. 1600
unbothered, unfazed, unruffled, indifferent
near-opposite of the traditional sense; arose in early 20th-c. American usage and is now widespread, but is still widely regarded as an error
Past-participle adjective of the verb nonplus, c. 1600, meaning "perplexed, puzzled, confounded." The verb (1590s) comes from the noun nonplus "a state in which one can go no further" (1580s), from Latin non plus "no more, no further" — the point at which nothing more can be said or done.
The traditional "bewildered" sense held remarkably stable from c. 1600 for roughly three centuries. A near-opposite sense, "unruffled, unconcerned," emerged in American usage in the early 20th century (Merriam-Webster cites attestations from 1930, 1948, and 1955), apparently from a mistaken assumption that non- is a negating prefix marking someone as other than "plussed." This newer sense has grown common enough to appear in well-edited publications yet is still categorically rejected by many as an error, making "nonplussed" a textbook contranym (Janus word).
Contranym / contested usage. The original and still-standard meaning is "bewildered, at a loss"; a newer chiefly North American informal sense means the near-opposite, "unfazed, unbothered." Merriam-Webster documents both, traces the "unruffled" sense to the early 20th century, and notes it "continues to be widely regarded as an error" even as it spreads. Treat the word with care: in ambiguous context the two senses are indistinguishable.