Lewis Short
(adjective) : spontālis, e, 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.
Charlton T. Lewis, Charles Short, A Latin Dictionary