An Etymological Dictionary of Astronomy and Astrophysics

Arising from internal forces or causes; independent of external agencies; self-acting.

Etymology (EN): From L.L. spontaneus “willing, of one’s free will,” from L. (sua) sponte “of one’s own accord, willingly,” of unknown origin.

Etymology (PE): Sarxod, literally “by himself/herself,” from sar “head” (soru, sorun “horn;”
karnâ “a trumpet-like wind instrument,” variant sornâ “a wind instrument;”
Mid.Pers. sar “head,” sru “horn;” Av. sarah- “head,” srū- “horn, nail;” cf. Skt. śiras- “head, chief;” Gk. kara “head,” karena “head, top,” keras “horn;”
L. cornu “horn,” cerebrum “brain;”
P.Gmc. *khurnaz (Ger. Horn, Du. horen; cognate with E. horn, as above, from PIE *ker- “head, horn;”
O.E. horn “horn of an animal,” also “wind instrument;”
E. horn); PIE base *ker- “head, horn, top, summit”) + xod “self” (Mid.Pers. xwad “self; indeed;”
Av. hva- “self, own”).