With her imperturbable calm she did not begin to speak in front of the valet.
Anatole was not quick-witted, nor ready or eloquent in conversation, but he had the faculty, so invaluable in society, of composure and imperturbable self-possession.
The elderly magnate was at first as much taken aback by this suggestion of marriage with a woman whose husband was alive, as the younger man had been, but Helene’s imperturbable conviction that it was as simple and natural as marrying a maiden had its effect on him too.
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Silas was imperturbable.
Neil Gaiman -- The Graveyard Book
Out of loudspeakers all around Zollverein, the staccato voice of the Reich grows like some imperturbable tree; its subjects lean toward its branches as if toward the lips of God.