Lewis & Short

exĭtĭābĭlis, e, adj. [exitium], destructive, fatal, deadly (rare but class.): exitiabilem illi faciam diem, Plaut. Ep. 4, 2, 36: bellum suis civibus, * Cic. Att. 10, 4, 3; so, bellum, Eutr. 9, 7: tyrannus, Liv. 29, 17, 19: clades, Suet. Aug. 23: discordiae, id. Claud. 25: reipublicae, Tac. H. 2, 69: morbus, id. A. 16, 5: fames, Vell. 2, 112, 3: telum, Ov. M. 6, 257: animus in suos, Tac. A. 6, 24: superstitio, id. ib. 15, 44.
Adv.: exĭtĭā-bĭlĭter, perniciously, fatally, August. Civ. D. 1, 17.