Lewis & Short

spontālis, e, adj. [sponte], voluntary (post-class.): parricidium, App. M. 4, p. 147, 25: sobrietas, id. ib. 11, p. 272, 30.
* Adv.: spontālĭter, voluntarily: facere (opp. coactus), Sid. Ep. 8, 9.