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Old English *willan, wyllan "to wish, desire, want" (past tense wolde), from Proto-Germanic *welljan (cf. Old Saxon willian, Old Norse vilja, Old Frisian willa, Dutch willen, Old High German wellan, German wollen, Gothic wiljan "to will, wish, desire," Gothic waljan "to choose"). The Germanic words are from PIE *wel-/*wol- "be pleasing" (cf. Sanskrit vrnoti "chooses, prefers," varyah "to be chosen, eligible, excellent," varanam "choosing;" Avestan verenav- "to wish, will, choose;" Greek elpis "hope;" Latin volo, velle "to wish, will, desire;" Old Church Slavonic voljo, voliti "to will," veljo, veleti "to command;" Lithuanian velyti "to wish, favor," pa-vel-mi "I will," viliuos "I hope;" Welsh gwell "better").
Cf. also Old English wel "well," literally "according to one's wish;" wela "well-being, riches." The use as a future auxiliary was already developing in Old English. The implication of intention or volition distinguishes it from shall, which expresses or implies obligation or necessity. Contracted forms, especially after pronouns, began to appear 16c., as in sheele for "she will." The form with an apostrophe is from 17c.
Old English will, willa, from Proto-Germanic *weljon (cf. Old Saxon willio, Old Norse vili, Old Frisian willa, Dutch wil, Old High German willio, German wille, Gothic wilja "will"), related to *willan "to wish" (see will (v.)). The meaning "written document expressing a person's wishes about disposition of property after death" is first recorded late 14c.