Unflagging

Adjective

 * 1) Never tiring or lacking energy; without rest; without slowing.
 *  His unflagging efforts have not gone unnoticed.

Synonyms

 * inexhaustible, tireless, untiring, unwearying, indefatigable

Thesaurus
ardent, assiduous, constant, continuing, diligent, dogged, enduring, energetic, faithful, fervent, going strong, hard, hardworking, immutable, in full force, in full swing, inalterable, indefatigable, indomitable, industrious, inexhaustible, insistent, invincible, laborious, lasting, loyal, never idle, never-tiring, obstinate, patient, patient as Job, permanent, perseverant, persevering, persistent, persisting, pertinacious, plodding, plugging, preoccupied, rapt, relentless, resolute, sedulous, single-minded, sleepless, slogging, stable, steadfast, steady, strenuous, stubborn, tenacious, tireless, unabated, unabating, unallayed, unbated, unconquerable, undaunted, undiminished, undiscouraged, undrooping, unexhausted, unfaded, unfailing, unfaltering, unflinching, unintermitting, uninterrupted, unnodding, unrelaxing, unrelenting, unremitting, unshaken, unsleeping, unsparing, unswerving, untiring, unwavering, unweakened, unweakening, unwearied, unwearying, unwinking, unwithered, unworn, utterly attentive, vehement, weariless, zealous

Etymology
un- + flag + -ing.