(a.) Lacking mercy; cruel; unsparing; as, a merciless tyrant; merciless waves\.

atrocious barbarian barbaric barbarous bestial bloody bowelless brutal brutish callous cold crude cruel cutthroat dog-eat-dog feral ferine ferocious fierce hard harsh heartless illiberal implacable inclement inconsiderate indifferent inexorable inflexible inhuman inhumane insensitive kill-crazy malevolent malign malignant mortal murderous noncivilized pitiless relentless remorseless rough rude ruthless sanguinary savage severe tameless thoughtless tough tyrannical unappeasable unbending uncalled-for uncharitable uncivilized uncompassionate uncompassioned unfeeling unflinching unforgiving ungentle ungracious unmerciful unmoved unpitiful unpitying unrelenting unremorseful unsparing unsympathetic unsympathizing untamed unyielding vicious wanton wild without mercy