The Collaborative International Dictionary of English v.0.48:

Unrelenting \Un`re*lent"ing\, a.
   Not relenting; unyielding; rigid; hard; stern; cruel. --
   Un`re*lent"ing*ly, adv. -- Un`re*lent"ing*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unrelenting
    adj 1: not to be placated or appeased or moved by entreaty;
           "grim determination"; "grim necessity"; "Russia's final
           hour, it seemed, approached with inexorable certainty";
           "relentless persecution"; "the stern demands of
           parenthood" [syn: grim, inexorable, relentless,
           stern, unappeasable, unforgiving, unrelenting]
    2: harsh; "the brutal summer sun"; "a brutal winter" [syn:
       brutal, unrelenting]
    3: never-ceasing; "the relentless beat of the drums" [syn:
       persistent, relentless, unrelenting]

Moby Thesaurus II by Grady Ward, 1.0:

137 Moby Thesaurus words for "unrelenting":
   adamant, adamantine, assiduous, cast-iron, constant, continuing,
   de longue haleine, diligent, dogged, dour, endless, enduring,
   extended, faithful, filled out, firm, flinty, fundamentalist, grim,
   hard, hard-core, hidebound, immovable, immutable, implacable,
   impliable, inalterable, indefatigable, indomitable, industrious,
   inelastic, inexorable, inflexible, insistent, intransigent,
   invincible, iron, ironbound, ironclad, ironhanded, irreconcilable,
   lasting, lengthy, long, long-drawn-out, long-spun, long-winded,
   longiloquent, loyal, merciless, mortal, muscle-bound, never-tiring,
   obdurate, obstinate, orthodox, padded, patient, patient as Job,
   permanent, perseverant, persevering, persistent, persisting,
   pertinacious, plodding, plugging, preoccupied, procrustean, prolix,
   protracted, purist, puristic, puritan, puritanic, rapt, relentless,
   resolute, rigid, rigorist, rigoristic, rigorous, rock-ribbed,
   rockbound, ruthless, sedulous, single-minded, sleepless, slogging,
   spun-out, stable, steadfast, steady, steely, stern, stiff,
   straightlaced, straitlaced, stubborn, talkative, tenacious,
   tireless, unabating, unaffected, unalterable, unappeasable,
   unbending, unchangeable, uncompromising, unconquerable, undaunted,
   undiscouraged, undrooping, unfailing, unfaltering, unflagging,
   unflinching, ungiving, unintermitting, uninterrupted, unmoved,
   unnodding, unrelaxing, unremitting, unsleeping, unswerving,
   untiring, unwavering, unwearied, unwearying, unwinking, unyielding,
   utterly attentive, verbose, weariless, windy, wordy