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English word ruthless comes from a combination of English ruth (an archaic term for sorrow for the misery of another; pity) and -less (lacking/without something)
ruth (English)
(archaic) Sorrow for the misery of another; pity, compassion; mercy. [from 13th c.]. (now, _, rare) Repentance; regret; remorse. [from 13th c.]. (obsolete) Something which causes regret or sorrow; a pitiful sight. [13th-17th c.]. (obsolete) Sorrow; misery; distress. [...]
-less (English)
Lacking (something); without (something).
ruthless (English)
Without pity or compassion; cruel, pitiless.