If you are nonplussed, something has surprised you so much that you stop. You do not know what to say or what to do next.

If you are如果你是 nonplussednonplussed, something has surprised you so much表示某件事把你嚇到了 that you stop讓你整個人愣住。You do not know你不知道 what to say接下來該說什麼 or what to do next也不知道該做什麼。

The old idea in the word is “no more”: you cannot go any further; you are stuck. Books, news, and teachers still use it this way.

The old idea in the word這個字裡舊有的想法 is是 "no more":「不能再多」: you cannot go any further你沒辦法再往前一步; you are stuck你卡住了。Books書, news新聞, and和 teachers老師 still use it this way還是這樣用這個字。

Though actually, in everyday American English many people now use it to mean the opposite — not bothered at all, as if nothing special happened. That newer use is probably from a misreading of non- as a simple “not,” so people guessed it meant “not affected.” Both uses are out there, so if someone says “I was nonplussed,” you may not know if they were lost for words or completely calm.

Though actually不過其實, in everyday American English在日常美式英語裡, many people很多人 now use it現在用它 to mean the opposite來表示相反的意思 — not bothered at all完全不受影響, as if好像 nothing special happened什麼特別的事都沒發生。That newer use這個比較新的用法 is probably from可能是來自 a misreading of non- as a simple "not,"把non-誤讀成單純的「不」, so所以 people guessed it meant人們就猜它的意思是 "not affected."「沒受影響」。Both uses are out there兩種用法都存在, so所以 if someone says如果有人說 "I was nonplussed,"「I was nonplussed」, you may not know你可能沒辦法確定 if they were lost for words他們是說不出話 or completely calm還是完全冷靜。

The older meaning sounds a little formal and bookish; the newer one is more casual US talk.

The older meaning比較舊的意思 sounds聽起來 a little formal and bookish有點正式、書面; the newer one比較新的那個 is more casual US talk比較像美式口語。