This is the word that got me started…dictionary writers are humorous.
Word Origin and History for spontaneous
adj. 1650s, from Late Latin spontaneus “willing, of one’s free will,” from Latin (sua) sponte “of one’s own accord, willingly;” of unknown origin. Related: Spontaneously. Earliest use is of persons and characters. Spontaneous combustion first attested 1795. Spontaneous generation (the phrase, not the event) attested from 1650s.