Merciless

wiktionary

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From Middle English merciles, mercyles, equivalent to mercy +‎ -less.

etymonline

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merciless (adj.)

late 14c., "unfeeling, pitiless, cruel," from mercy + -less. Sense of "relentless" is from early 15c.; of inanimate things, from 1580s. Related: Mercilessly; mercilessness.