callous

callous adjective adamant, adamantine, brutal, callosus, cold, cold of heart, coldhearted, durus, hard, hard of heart, hardened, hardhearted, impassive, impenetrable, impenitent, imperturbable, impervious, implacable, inclement, indifferent, indisposed to mercy, indurate, indurated, inexcitable, inexorable, infrangible, insensate, insensible, insensitive, intolerant, obdurate, persecuting, pitiless, relentless, remorseless, rigorous, seared, severe, shock-proof, steeled against, stubborn, tearless, thick-skinned, unbending, unblushing, uncaring, uncompassionate, uncondoling, unfeeling, unforgiving, unimpressed, unimpressible, unmelting, unmerciful, unmoved, unpardoning, unrelenting, unruffled, unstirred, unsusceptible, unsympathetic, unsympathizing, unyielding

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See also: cold-blooded, diabolic, impervious, insensible, insusceptible, obdurate, obtuse, phlegmatic, recalcitrant, ruthless, unaffected