The Collaborative International Dictionary of English v.0.48:

Ruthless \Ruth"less\, a.
   Having no ruth; cruel; pitiless.
   [1913 Webster]

         Their rage the hostile bands restrain,
         All but the ruthless monarch of the main. --Pope.
   [1913 Webster] -- Ruth"less*ly, adv. -- Ruth"less*ness,
   n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ruthless
    adj 1: without mercy or pity; "an act of ruthless ferocity"; "a
           monster of remorseless cruelty" [syn: pitiless,
           remorseless, ruthless, unpitying]

Moby Thesaurus II by Grady Ward, 1.0:

90 Moby Thesaurus words for "ruthless":
   Draconian, Tartarean, animal, anthropophagous, atrocious, barbaric,
   barbarous, beastly, bestial, bloodthirsty, bloody, bloody-minded,
   bowelless, brutal, brutalized, brute, brutish, callous,
   cannibalistic, cruel, cruel-hearted, demoniac, demoniacal,
   devilish, diabolic, dog-eat-dog, fell, feral, ferine, ferocious,
   fiendish, fiendlike, fierce, flinty, hard, harsh, heartless,
   hellish, implacable, inclement, inexorable, infernal, inhuman,
   inhumane, kill-crazy, malign, malignant, mean, merciless, mortal,
   murderous, noncivilized, pitiless, relentless, remorseless,
   sadistic, sanguinary, sanguineous, satanic, savage, severe,
   sharkish, slavering, subhuman, tameless, tough, truculent,
   unappeasable, unchristian, uncivilized, uncompassionate,
   uncompassioned, unfeeling, unflinching, unforgiving, ungentle,
   unhuman, unmerciful, unpitiful, unpitying, unrelenting,
   unremorseful, unsympathetic, unsympathizing, untamed, unyielding,
   vicious, wild, without mercy, wolfish