spontaneous

English borrowed 'spontaneous' as a learned term from Late Latin spontāneus, which was built from Latin sponte, meaning 'of one's own free will,' plus a suffix that turned it into an adjective describing that quality.

The facts don't give us a record of shifting meaning over the centuries — only that it entered English directly as a learned borrowing from Late Latin, already carrying its sense of unforced, self-originating action.

The facts don't support naming a specific English cognate beyond the word itself and its Latin source, sponte.