Lewis & Short

imprōvīsus (inpr-), a, um, adj. [2. in-provisus],

  1. I. not foreseen, unforeseen, unexpected (class.): sapienti nihil improvisum accidere potest, nihil inopinatum, nihil omnino novum, Cic. Tusc. 4, 15, 37; id. Verr. 2, 2, 28, § 69; id. de Or. 1, 22, 103: unde iste amor tam improvisus ac tam repentinus? id. Agr. 2, 22, 60: cum tot bella subito atque improvisa nascantur, id. Font. 15, 32: mala, id. Tusc. 3, 14, 30: improvisior pestis, Tac. A. 2, 47: castella munita improviso adventu capta, Cic. Fam. 2, 10, 3: pericula, id. Mur. 27, 55: vis leti, Hor. C. 2, 13, 19: species, id. Ep. 1, 6, 11: pelagoque remenso Improvisi aderunt, Verg. A. 2, 182: cunctisque repente Improvisus ait, id. ib. 1, 595.
  2. II. As subst.: imprōvīsum, i, n., that which is unforeseen, an emergency: dux sibi delectos retinuerat ad improvisa, Tac. H. 5, 16.
    Mostly in abl., with de, adverbially, unexpectedly, suddenly: quasi de improviso respice ad eum, Ter. And. 2, 5, 6; cf. Trabea ap. Cic. Tusc. 4, 31, 67; Cic. Rosc. Am. 52, 151; cf. also Caes. B. C. 2, 6, 3: de improviso, Ter. And. 2, 2, 23; id. Heaut. 2, 3, 40; id. Ad. 3, 3, 53; 4, 4, 1; id. Phorm. 5, 6, 44; Caes. B. G. 2, 3, 1; 5, 22, 1; 5, 39, 1; 6, 3, 1 et saep.
    Less freq. with ex: ex improviso filiam inveni meam, Plaut. Rud. 4, 5, 2; Cic. Verr. 2, 1, 43, § 112.
    Hence, adv., in two forms, imprōvīso (class.) and imprōvīsē (post-class.), on a sudden, unexpectedly: improviso filiam inveni (for which, shortly before: ex improviso filiam inveni), Plaut. Rud. 4, 5, 6: sane homini praeter opinionem improviso incidi, Cic. Verr. 2, 2, 74, § 182: improviso eos in castra irrupisse, id. Div. 1, 24, 50: tantum adest boni improviso, Plaut. As. 2, 2, 44: cum mihi nihil improviso evenisset, Cic. Rep. 1, 4; id. Att. 15, 1, A, 1: scalae improviso, subitusque apparuit ignis, Verg. A. 12, 576; 8, 524: improvise necans incautos morte suprema, Tert. poët. adv. Marc. 2, 12.