nonplus
Function: transitive verb
Inflected Form(s):
non·plussed also non·plused \-ˈpləst\; non·plus·sing also non·plus·ing \-ˈplə-siŋ\
Date: 1591
: to cause to be at a loss as to what to say, think, or do : perplex

(from a Sunday Times' forum letter)