nonplussed

First recorded in 1600–10; nonplus + -ed²

The most common sense of nonplussed is “puzzled or perplexed”: Her odd choice of words left me somewhat nonplussed. But there is a more recent and less common usage with almost the opposite meaning, “unfazed, indifferent, or unexcited”: She remained nonplussed throughout the child’s tantrum, simply waiting till it was over. Although this second usage is gaining traction even in edited writing, many speakers and writers consider it nonstandard and avoid it as potentially confusing.
There are several theories about how this contradictory second meaning came into being. One explanation is the analysis of nonplussed as non- “not” + plussed, interpreted as “not puzzled, perplexed, bothered, or impressed,” possibly by analogy with other words that have a negative prefix and similar meaning, such as unfazed and indifferent, and perhaps influenced by nonchalant. However, plussed has no meaning in English, and the non at the beginning is not in fact the common English negative prefix non- found in such words as nonalcoholic or nonexistent. Interestingly, the adjective nonplussed was formed from the less common verb nonplus “to puzzle or perplex,” which was originally a noun meaning “a state of utter perplexity,” and this noun was borrowed from the Latin phrase nōn plūs “not more; no further.” In other words, a state of perplexity is one in which the person can think of no more to say or do.