nonplussed
Nonplussed (adj) means So confused or taken aback that you do not know what to do.
Etymology
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”.