Old English nylle, nelle "to be unwilling," from ne "no" (from PIE root *ne- "not") + will (v.). Often paired with will; the once-common construction nill he, will he, attested from c. 1300, survives principally in willy-nilly, which, however, reverses the usual Middle English word order. Latin expressed a similar idea in nolens volens.
Entries linking to nill
It forms all or part of: a- (3) "not, without;" abnegate; ahimsa; an- (1) privative prefix; annihilate; annul; aught (n.2) "zero, nothing;" deny; hobnob; in- (1) "not, opposite of, without;" ixnay; naught; naughty; nay; nefarious; negate; neglect; negligee; negotiate; neither; nepenthe; nescience; nescient; neuter; never; nice; nihilism; nihility; nil; nill; nimiety; nix; no; non-; none; nonplus; nor; not; nothing; null; nullify; nulliparous; renegade; renege; un- (1) prefix of negation; willy-nilly.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit a-, an- "not;" Avestan na "not;" Greek a-, an-, ne- "not;" Latin in- "not," ne "that not;" Old Church Slavonic and Lithuanian ne "not;" Old Irish an-, ni, Cornish ny "not;" Gothic and Old English un- "not."
The Germanic words are from PIE root *wel- (2) "to wish, will" (source also of 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-velmi "I will," viliuos "I hope;" Welsh gwell "better").
Compare 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." In early use often -ile to preserve pronunciation. The form with an apostrophe ('ll) is from 17c.