Not easily perturbed or excited or upset; marked by extreme calm and composure:
— Hitherto imperturbable, he now showed signs of alarm.
— An imperturbable self-possession.

synonym: unflappable.

Roget 823: insensible, unconscious; impassive, impassible; blind to, deaf to, dead to; unsusceptible, insusceptible; unimpressionable^†, unimpressible^†; passionless, spiritless, heartless, soulless; unfeeling, unmoral.    apathetic; leuco-^†, phlegmatic; dull, frigid; cold blooded, cold hearted; cold as charity; flat, maudlin, obtuse, inert, supine, sluggish, torpid, torpedinous^†, torporific^†; sleepy etc. (inactive) 683; languid, half-hearted, tame; numbed; comatose; anaesthetic etc. 376; stupefied, chloroformed, drugged, stoned; palsy-stricken.    indifferent, lukewarm; careless, mindless, regardless; inattentive etc. 458; neglectful etc. 460; disregarding.    unconcerned, nonchalant, pococurante^†, insouciant, sans souci [Fr.]; unambitious etc. 866.    unaffected, unruffled, unimpressed, uninspired, unexcited, unmoved, unstirred, untouched, unshocked^†, unstruck^†; unblushing etc. (shameless) 885; unanimated; vegetative.    callous, thick-skinned, hard-nosed, pachydermatous, impervious; hardened; inured, casehardened; steeled against, proof against; imperturbable etc. (inexcitable) 826^†; unfelt.    ... show more

Roget 826: inexcitable^†, unexcitable; imperturbable; unsusceptible etc. (insensible) 823; unpassionate^†, dispassionate; cold-blooded, irritable; enduring etc. v.; stoical, Platonic, philosophic, staid, stayed; sober, sober minded; grave; sober as a judge, grave as a judge; sedate, demure, cool-headed.    easy-going, peaceful, placid, calm; quiet as a mouse; tranquil, serene; cool as a cucumber, cool as a custard; undemonstrative.    temperate etc. (moderate) 174; composed, collected; unexcited, unstirred, unruffled, undisturbed, unperturbed, unimpassioned; unoffended^†; unresisting.    meek, tolerant; patient, patient as Job; submissive etc. 725; tame; content, resigned, chastened, subdued, lamblike^†; gentle as a lamb; suaviter in modo [Lat.]; mild as mothers milk; soft as peppermint; armed with patience, bearing with, clement, long-suffering.    ... show more