Imperturbable, a.
Etymology: [L. imperturbabilis; pref. im- not + perturbare to disturb: cf. F. imperturbable. See Perture.]
Incapable of being disturbed or disconcerted; as, imperturbable gravity.
Shades of meaning
Fernald has no entry of its own for imperturbable; he lists it under calm.
He lists it without comment; the entries above explain its neighbors.