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Definition of relentless : Unmoved by appeals for sympathy or forgiveness; insensible to the distresses of others; destitute of tenderness; unrelenting; unyielding; unpitying; as, a prey to relentless despotism.

Synonyms:

hard, low, everlasting, continuous, perpetual, continual, downhearted, stiff, disconsolate, unmitigated, adamant, die-hard, drear, haunting, dingy, uninterrupted, eternal, dispirited, dismal, unrelenting, unending, unfailing, brassbound, around-the-clock, ongoing, morose, drab, ceaseless, iron, intransigent, mordant, cruel, remorseless, round-the-clock, sick, endless, incompliant, macabre, interminable, black, resist, low-spirited, uncompliant, downcast, nonstop, grisly, blue, ghastly, gloomy, sorry, brutal, unremitting, dark, unmerciful, adamantine, incessant, unappeasable, unbendable, dreary, depressed, lasting, gruesome, down, forbidding, continue, unforgiving, timeless, unceasing, down in the mouth, pertinacious

hearty, indefatigable, iron-willed, committed, reliable, determined, faithful, true, loyal, resolved, bold, stubborn, valiant, thorough, willing, unyielding, dedicated, unchangeable, courageous, deliberate, decided, tough, decisive, tireless, implacable, headstrong, devoted, plucky, intent, steely, unshakeable, persistent, diligent, spunky, steady, strong-minded, bullheaded, earnest, strong-willed, tenacious, steadfast, constant, inexorable, persevering, firm, zealous, staunch, ruthless, unwavering, unflinching, fearless, dogged, resolute, insistent, serious, dauntless, indomitable, set

demanding, rigid, austere, fundamental, gruff, rigorous, brusque, dry, uncompromising, piquant, curt, oppressive, correct, brisk, chilly, obdurate, icy, stiff-necked, bleak, meticulous, disciplined, cutting, spare, stringent, hidebound, blunt, caustic, censorious, keen, cool, tart, exacting, basic, acute, sharp, ascetic, acerbic, imperial, puritanical, astringent, precise, lean, severe, frosty, Spartan, Draconian, harsh, acrimonious, unbending, intense, prudish, obstinate, intolerant, critical, authoritarian, short, abrupt, strict, stern, raw, stark, inflexible, crisp, grim, strait-laced, dour

Usage examples:

• That was what Lund would be from now on, a driver, imperative, relentless, overcoming all obstacles; as he had himself said, selfish at heart, keen for his own ends. - "A Man to His Mate", J. Allan Dunn.
• But Betsy was relentless. - "Understood Betsy", Dorothy Canfield.
• It was like the rising of a tide; it was determined, almost relentless in its appearance, but it was slow. - "Last Words", Stephen Crane.