Ruth•less Adjective Having or showing no pity or compassion for others. "A ruthless manipulator" Synonyms: merciless, pitiless, cruel, heartless, hard-hearted, cold-hearted, cold blooded, harsh, callous, unmerciful, unforgiving, uncaring, unsympathetic, uncharitable, remorseless, unbending, inflexible, implacable; brutal, inhuman, barbarous, barbaric, savage, sadistic. "Ruthless killers" "Harry you are completely ruthless." I whisper. "I hate you!!" I scream in anger. I hate him, and I've never felt so much hatred for another being.