Noah Webster's Dictionary (a.) Unmoved by appeals for sympathy or forgiveness; insensible to the distresses of others; destitute of tenderness; unrelenting; unyielding; unpitying; as, a prey to relentless despotism.

Multi-Version Concordance Relentless (1 Occurrence) Isaiah 14:6 He that smote the peoples in wrath with a relentless stroke, he that ruled the nations in anger, is persecuted unsparingly. (DBY NIV)