spontaneous adj.

1650s, "occurring without external stimulus," from Late Latin spontaneus "willing, of one's free will," from Latin (sua) sponte "of one's own accord, willingly," a word of uncertain origin. Related: Spontaneously; spontaneousness. Used earlier of persons and characters, with a sense "acting of one's own accord" (c. 1200). Spontaneous combustion first attested 1795. Spontaneous generation (the phrase, not the feat) attested from 1650s.