Meaning

adjective happening or arising without apparent external cause

adjective said or done without having been planned or written in advance

originEtymology tree Late Latin sponte Proto-Indo-European *-nós Proto-Italic *-nos Late Latin -nus Late Latin -ānus Proto-Indo-European *-yósder. Proto-Indo-European *-éyos Proto-Italic *-ejos Proto-Italic *-eos Late Latin -eus Late Latin -āneus Late Latin spontāneuslbor. English spontaneous Learned borrowing from Late Latin spontāneus.

Etymology from Wiktionary, CC BY-SA 3.0