Unexcitable

Ordinarily the commandant of the fort was of a calm and unexcitable temperament. During the astounding events of that day and the day before he had kept his head cool; his judgment, if not correct, was the result of sober and earnest consideration.

[…] he remembered how terse and unexcitable the jeweller was, so different from those gem-dealers who, meeting with each other on the pavements at Hatton Garden, could not contain for two seconds their business verve, nor refrain from displaying there and then their tiny precious wares, produced out of waistcoat pockets and wrapped in tissue paper.

But suppose that these pretended living Stones of the Temple of God were as unexcitable to Pride or Lust as the dead Stones of the walls of a Church […]

1794, Erasmus Darwin, Zoonomia, London: J. Johnson, Volume I, Section 12, 2. Of sensorial Exertion, p. 79, When the spirit of animation is thus exhausted by useless exertions, the organ becomes torpid or unexcitable into action, and a second fit of quiescence succeeds that of abundant activity.

Isabella […] had been misled, as most inexperienced observers are in similar cases, by the tranquillity of Eliot’s manner; she respected and liked him exceedingly; but she thought him unexcitable, and incapable of passion.