Willfully

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Middle English: from the noun will2 + -ful.

wiktionary

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willful +‎  -ly

etymonline

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willfully (adv.)

also wilfully, late Old English wilfullice "of one's own free will, voluntarily;" see willful + -ly (2). Mid-14c. as "deliberately, knowingly." Bad sense of "on purpose" is attested from late 14c.