From Christopher Tolkien's glossary to Chaucer's Nun's Priest's Tale:
Medieval English possessed special negative forms of some common verbs; see nys, nas, nere, noot [ nys from ne is, is not; nas from ne was, was not; nere from ne were, were it not; noot from ne woot, I do not know]...
The phrase 'willy nilly' still contains one: 'will I, nill I' or whether I wish it or wish it not.
The usage appears in Arwen's tragic words beside Aragorn's death bed:
I must indeed abide the Doom of Men, whether I will or I nil: the loss and the silence.