The Collaborative International Dictionary of English v.0.48:

Unflagging \Unflagging\
   See flagging.

WordNet (r) 3.0 (2006):

unflagging
    adj 1: showing sustained enthusiastic action with unflagging
           vitality; "an indefatigable advocate of equal rights"; "a
           tireless worker"; "unflagging pursuit of excellence"
           [syn: indefatigable, tireless, unflagging,
           unwearying]
    2: unceasing; "unfailing loyalty"; "unfailing good spirits";
       "unflagging courtesy" [syn: unfailing, unflagging]

Moby Thesaurus II by Grady Ward, 1.0:

91 Moby Thesaurus words for "unflagging":
   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