Lewis & Short

ĭn-ŏpīnans, antis, adj., not expecting, unaware (freq. in the historians; not in Cic.): inscios inopinantesque Menapios oppresserunt, Caes. B. G. 4, 4: impeditos et inopinantes aggressus, id. ib. 1, 12: imprudente atque inopinante Curione, id. B. C. 2, 3: rex in potestatem inopinanti (ei) venerat, Nep. Dat. 3, 4: suis inopinantibus, Liv. 27, 48, 14.
Hence, adv.: ĭnŏpīnanter, unexpectedly, Suet. Tib. 60.