The Collaborative International Dictionary of English v.0.48:

Untiring \Untiring\
   See tiring.

WordNet (r) 3.0 (2006):

untiring
    adj 1: characterized by hard work and perseverance [syn:
           hardworking, industrious, tireless, untiring]

Moby Thesaurus II by Grady Ward, 1.0:

90 Moby Thesaurus words for "untiring":
   assiduous, cartilaginous, chewy, cohesive, constant, continuing,
   coriaceous, dedicated, determined, diligent, dogged, durable,
   enduring, faithful, fibrous, gristly, hard, hardy, immutable,
   inalterable, indefatigable, indomitable, industrious,
   inexhaustible, insistent, invincible, lasting, leatherlike,
   leathery, loyal, never-tiring, obstinate, patient, patient as Job,
   permanent, perseverant, persevering, persistent, persisting,
   pertinacious, plodding, plugging, preoccupied, rapt, relentless,
   resistant, resolute, ropy, sedulous, sinewy, single-minded,
   sleepless, slogging, stable, steadfast, steady, stiff, stringy,
   strong, stubborn, tenacious, tireless, tough, tough as leather,
   unabating, unconquerable, undaunted, undiscouraged, undrooping,
   unfailing, unfaltering, unflagging, unflinching, unintermitting,
   uninterrupted, unnodding, unrelaxing, unrelenting, unremitting,
   unsleeping, unswerving, unwavering, unwearied, unwearying,
   unwinking, utterly attentive, vigorous, viscid, weariless, wiry