Lewis & Short

exĭtĭōsus, a, um, adj. [exitium], destructive, pernicious, deadly (rare but class.): conjuratio, Cic. Cat. 4, 3, 6: quibus a servis caedem fieri senatus et bonorum rei publicae exitiosum fuisset, id. Planc. 36, 87; cf.: quod exitiosum fore, si evenisset, videbam, id. Fam. 6, 1, 5.
Of persons (post-Aug.): rex, Tac. A. 6, 36; id. H. 1, 68.
Comp.: Otho luxu, saevitia, audacia reipublicae exitiosior ducebatur, Tac. H. 2, 31.
Sup., Tert. Anim. 34.
Adv.: ex-ĭtĭōsē, perniciously.
Sup.,
Aug. Ep. 8, 3.