means happening naturally and without planning, prompted by an inner impulse rather than outside force or forethought

from From Late Latin spontaneus, 'willing, of one's own accord,' built on the Latin phrase sua sponte, 'of one's own free will.' That sponte traces back to spons, a word for free will or inclination that survives almost only in this fossilized phrase — so the whole idea of spontaneity is essentially carried by a single nearly-extinct Latin noun for acting because you simply wanted to.