Philistine, adamant, adamantine, bony, bowelless, callous, calloused, case-hardened, cast-iron, cement, cemental, concrete, constant, corneous, cruel, dense, diamondlike, dog-eat-dog, dour, dure, firm, fixed, flintlike, granitelike, granitic, grim, hard-core, hard, hard as nails, hardened, hardhearted, harsh, heartless, horny, immovable, immutable, impervious, implacable, impliable, inclement, indurated, inelastic, inexorable, inflexible, insensitive, intransigent, inured, iron-hard, iron, ironlike, irreconcilable, lapideous, lithic, lithoid, lithoidal, loyal, marble, marblelike, merciless, obdurate, osseous, pachydermatous, petrified, petrogenic, pitiless, proof against, relentless, remorseless, resistant, resistive, rigid, rigorous, rock-ribbed, rock, rocklike, rocky, ruthless, set, settled, slaty, solid, sot, staunch, steadfast, steady, steeled against, steellike, steely, stern, stiff, stone, stonelike, stony, thick-skinned, tough, true, unaffected, unalterable, unbending, unchangeable, uncompassionate, uncompassioned, uncompromising, undeflectable, unfeeling, unflappable, unforgiving, ungiving, unmerciful, unmoved, unpitiful, unpitying, unrelenting, unremorseful, unshaken, unswerving, unsympathetic, unsympathizing, unyielding, without mercy