Spontaneous

google

ref

mid 17th century: from late Latin spontaneus (from ( sua) sponte ‘of (one's) own accord’) + -ous.

wiktionary

ref

Late Latin spontāneus, from Latin sponte (suā)(“of one's free will, voluntarily”).

etymonline

ref

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.