Imperturbable

[T]o a vviſe man there can happen no iniury or offence at all, to moleſt the felicity of his minde, vvhich (in the Stoicks opinion) ought to bee imperturbable, and his heart adamantine.

[T]he good Dominie bore all his disasters with gravity and serenity equally imperturbable. "Prodi-gi-ous!" was the only ejaculation they ever extorted from the much-enduring man.

In the meanwhile, Sir Piercie Shafton sate still as a stone on the chair in which he had deposited himself, […] his eyes cast up to the ceiling as if he had meant to count every mesh of cobweb with which the arched roof was canopied, wearing at the same time a face of as solemn and imperturbable gravity, as if his existence had depended on the accuracy of his calculation.

"We may keep it by us," replied the pawnbroker, "for months; there is no demand for such articles." "But," exclaimed she, eagerly, "I shall soon redeem it!" "So you all say," returned the man, with imperturbable coolness.

This sort of thing is meat and drink to the born Controller—and Controllers are born with the right imperturbable temperament for the job; hence the fact that they are recruited from many different grades of operating staff, and some recruits don't stay the course.