Ruthless

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Middle English: from ruth + -less.

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From Middle English reuþeles; equivalent to ruth +‎ -less.

etymonline

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ruthless (adj.)

early 14c., reutheles, "pitiless, merciless, devoid of compassion," from reuthe "pity, compassion" (see ruth) + -less.

Ruthful "pitiable, lamentable, causing ruth" (c. 1200) has fallen from use since late 17c. except as a deliberate archaism, perhaps in part because it had a conflicting sense of "compassionate, tender-hearted, full of ruth." Ruthness "compassion, pity" (early 14c.) died even younger. Related: Ruthlessly; ruthlessness.