non-plus

tr.v. non-plused also non-plussed, non-plus-ing non-plus-sing, non-plus-es non-plus-ses.
1. To put at a loss as to what to think, say, or do; bewilder. --non-plus n. A state of perplexity, confusion, or bewilderment. [From Latin non plus, no more : non, not; see NON- + plus.

Source: American Heritage Dictionary