nonplus
Concise Oxford English Dictionary © 2008 Oxford University Press:
nonplus /nɒnˈplʌs/
▶verb (nonplusses, nonplussing, nonplussed)
- 1 surprise and confuse; flummox.
- 2 (as adj. nonplussed) N. Amer. informal unperturbed.
– origin C16: from L. non plus ‘not more’.
'nonplus' also found in these Oxford entries:

