nonplussed

From nonplus (“state of bewilderment or perplexity”, noun) or nonplus (“to bewilder or perplex (someone)”, verb) + -ed (suffix forming adjectives, and the past tense and past participle forms of verbs). Nonplus (noun) is derived from Latin nōn plūs (“no further, no more”), from nōn (“not”) + plūs (“additionally, more; further”) (ultimately from Proto-Indo-European *pleh₁- (“to fill”)). The etymological sense is similar to being left speechless as a result of confusion: the nonplussed person can say or do “no more”. Adjective sense 2 (“unaffected”) is probably from a misinterpretation of the first element of the word as the prefix non- meaning “not”.

来源：wiktionary