Etymology
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relentless (adj.)

"incapable of relenting, unmoved by pity, unpitying, insensible to the distress of others," 1590s, from relent + -less. Related: Relentlessly; relentlessness. Relenting is from 1590s as a present-participle adjective, "inclined to relent; too easily moved." For a noun for the group, relentment "act or state of relenting" (1620s) has been tried.

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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.

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hard-nosed (adj.)
"stubborn," 1927, from hard (adj.) + nose (n.). Earlier of bullets or shells with hard tips, and of dogs that had difficulty following a scent. Not in common use before 1950s, when it begins to be applied to tough or relentless characters generally (Damon Runyon characters, U.S. Marines, Princeton professors, etc.). Soft-nosed seems to have been used only of bullets.
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