etymologeek.com dictionary
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).

Ruthless etymology history?

Ruthless detailed word origin explanation

Dictionary entryLanguageDefinition
ruth English (eng) (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 (eng) Lacking (something); without (something).
ruthless English (eng) Without pity or compassion; cruel, pitiless.

Words with the same origin as ruthless

Descendants of -less
breathless clueless countless defenseless fearless flawless hopeless meaningless merciless needless penniless pointless priceless regardless relentless selfless senseless sleepless thoughtless timeless toothless topless useless wireless