Definition of "imperturbability"

"As I sat alone at my breakfast--Mrs. Challenger is in the habit of keeping her room of a morning--it suddenly entered my head that it would be entertaining and instructive to see whether I could find any limits to this woman's imperturbability."

1913, Arthur Conan Doyle, “(please specify the page)”, in The Poison Belt […], London; New York, N.Y.: Hodder and Stoughton