Impassibility, a.
Etymology: [L. impassibilitas: cf. F. impassibilité.]
The quality or condition of being impassible; insusceptibility of injury from external things.
Shades of meaning
Fernald has no entry of its own for impassibility; he lists it under apathy.
Where he writes about it:
Impassibility is a philosophical term applied to the Deity, as infinitely exalted above all stir of passion or emotion. — from apathy