non·plus (nŏn-plŭs') tr.v.
non·plussed also non·plused, non·plus·sing also non·plus·ing, non·plus·ses also non·plus·es To put at a loss as to what to think, say, or do; bewilder. n. A state of perplexity, confusion, or bewilderment.
[From Latin nōn plūs, no more : nōn, not; see non- + plūs, more; see pelə-1 in Indo-European roots.]
1582 (n.), properly "state where 'nothing more' can be done or said," from L. non plus "no more, no further." The verb meaning "to bring to a nonplus, to perplex" is attested from 1591.