Impassivity

His impassivity had been penetrated at last; it could no longer hide a nervousness and apprehension which kept his head perpetually pivoting in backward glances across his shoulder.

He wore gold-rimmed pince-nez, and had a curious impassivity of feature. It struck me that he might look natural on a stage, but was strangely out of place in real life.

[T]hey were baffled by tears in moustached sixth-formers, by walls of impassivity in the Lower School, by silent conspiracies which nullified the rules.