Etymology
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nonplussed (adj.)

"perplexed, puzzled, confounded," c. 1600, past-participle adjective from nonplus, which is from Latin non plus "no more, no further." 

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nonplus (v.)

"to bring to a nonplus, to perplex, puzzle, confound," 1590s, from the noun nonplus "state in which one is unable to proceed or decide" (1580s), usually in a phrase such as at or to a nonplus, properly "state where 'nothing more' can be done or said," from Latin non plus "no more, no further" (see plus). Related: Nonplussed.

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