See relentless in Webster's New World Roget's A-Z Thesaurus II
adjective
Firmly, often unreasonably immovable in purpose or will:
adamant, adamantine, brassbound, die-hard, grim, implacable, incompliant, inexorable, inflexible, intransigent, iron, obdurate, remorseless, rigid, stubborn, unbendable, unbending, uncompliant, uncompromising, unrelenting, unyielding. Idiom: stubborn as a mule (or ox). See resistExisting or occurring without interruption or end:
around-the-clock, ceaseless, constant, continual, continuous, endless, eternal, everlasting, incessant, interminable, nonstop, ongoing, perpetual, persistent, round-the-clock, timeless, unceasing, unending, unfailing, uninterrupted, unremitting. See continue
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