Stubborn

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Middle English (originally in the sense ‘untameable, implacable’): of unknown origin.

wiktionary

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From Middle English stiborne, stibourne, stoburn, stoburne, styburne, stiborn. Origin uncertain.

One theory is that the origin may come from *stybor, *stibor, from Old English stybb(“a stump, stub”) + adj. formative -or as in Old English bitor, English bitter.

etymonline

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stubborn (adj.)

late 14c., of uncertain origin. Earliest form is stiborn. OED, Liberman doubt any connection with stub (n.). Related: Stubbornly; stubbornness.