Synonyms and related words :
adamant, adamantine, ardent, assiduous, bound, bound and determined, bowelless, cast-iron, ceaseless, certain, committed, constant, continual, continuing, continuous, cruel, decided, decisive, dedicated, definite, determined, devoted, diligent, dog-eat-dog, dogged, dour, earnest, enduring, energetic, faithful, fated, fateful, ferocious, fervent, fierce, firm, flinty, fundamentalist, grim, habitual, hard, hard-core, hardworking, harsh, heartless, hidebound, immovable, immutable, implacable, impliable, inalterable, incessant, inclement, indefatigable, indefeasible, indomitable, industrious, inelastic, ineluctable, inescapable, inevasible, inevitable, inexorable, inflexible, inhuman, insistent, intractable, intransigent, invincible, iron, ironbound, ironclad, ironhanded, irreconcilable, irresistible, irrevocable, laborious, lasting, loyal, merciless, mortal, muscle-bound, necessary, never idle, never-tiring, obdurate, obstinate, orthodox, patient, patient as Job, permanent, perpetual, perseverant, persevering, persistent, persisting, pertinacious, pitiless, plodding, plugging, preoccupied, procrustean, purist, puristic, puritan, puritanic, purposeful, rapt, regular, remorseless, resistless, resolute, resolved, rigid, rigorist, rigoristic, rigorous, rock-ribbed, rockbound, ruthless, sedulous, serious, sincere, single-minded, sleepless, slogging, stable, steadfast, steady, steely, stern, stiff, stiff-necked, straightlaced, straitlaced, strenuous, strict, stringent, stubborn, sure, sure as death, sure as fate, tenacious, tireless, tough, unabated, unabating, unaffected, unalterable, unappeasable, unavoidable, unbending, unbroken, unceasing, unchangeable, uncompassionate, uncompassioned, uncompromising, unconquerable, uncontrollable, undaunted, undeflectable, undeviating, undiscouraged, undrooping, unfailing, unfaltering, unfeeling, unflagging, unflinching, unforgiving, ungiving, unintermitting, uninterrupted, unmerciful, unmoved, unmoving, unnodding, unpitiful, unpitying, unpreventable, unrelaxing, unrelenting, unrelieved, unremitting, unremorseful, unsleeping, unsparing, unstoppable, unswerving, unsympathetic, unsympathizing, untiring, unwavering, unwearied, unwearying, unwinking, unyielding, utterly attentive, vehement, weariless, wholehearted, without mercy, zealous