Willing, a.
Etymology: [From Will, v. t.]
1. Free to do or to grant; having the mind inclined; not opposed in mind; not choosing to refuse; disposed; not averse; desirous; consenting; complying; ready. Felix, willing to show the Jews a pleasure, left Paul bound. Acts xxiv. 27. With wearied wings and willing feet. Milton. [Fruit] shaken in August from the willing boughs. Bryant.
2. Received of choice, or without reluctance; submitted to voluntarily; chosen; desired. [They] are held, with his melodious harmony, In willing chains and sweet captivity. Milton.
3. Spontaneous; self-moved. [R.] No spouts of blood run willing from a tree. Dryden.
Shades of meaning
Fernald has no entry of its own for willing; he lists it under spontaneous.
Where he writes about it:
That is spontaneous which is freely done, with no external compulsion and, in human actions, without special premeditation or distinct determination of the will; that is voluntary which is freely done with distinct act of will; that is involuntary which is independent of the will, and perhaps in opposition to it; a willing act is not only in accordance with will, but with desire. — from spontaneous
We speak of spontaneous generation, spontaneous combustion, spontaneous sympathy, an involuntary start, an unbidden tear, voluntary agreement, willing submission. — from spontaneous