Definition of "imperturbable"

"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.

1837, L[etitia] E[lizabeth] L[andon], “Poverty”, in Ethel Churchill: Or, The Two Brides. […], volume III, London: Henry Colburn, […], page 280

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.

1962 August, G. Freeman Allen, “Traffic control on the Great Northern Line”, in Modern Railways, page 132