Voluntary

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late Middle English: from Old French volontaire or Latin voluntarius, from voluntas ‘will’.

wiktionary

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From Middle English *voluntarie, from Old French volontaire, from Latin voluntārius(“willing, of free will”), from voluntās(“will, choice, desire”), from volēns, present participle of volo(“to will”).

etymonline

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voluntary (adj.)

late 14c., from Latin voluntarius "willing, of one's free will," from voluntas "will," from the ancient accusative singular present participle of velle "to wish" (see will (v.)). Originally of feelings, later also of actions (mid-15c.). Related: Voluntarily.