Lewis & Short

ăd-ŏpĕrĭo, ĕrŭi, ertum, 4, v. a., to cover up or over (not used before the Aug. per., and gen. in the part. perf. pass.): capite adoperto, Liv. 1, 26; id. Epit. 89, and Suet. Ner. 48: purpureo adopertus amictu, Verg. A. 3, 405: tempora adoperta cucullo, Juv. 8, 145: adopertam floribus humum, Ov. M. 15, 688; cf. id. ib. 8, 701: hiems gelu, id. F. 3, 235: aether nubibus, id. ib. 2, 75: lumina somno, id. M. 1, 714: tenebris mors, Tib. 1, 1, 70: foribus adopertis, with closed doors, Suet. Oth. 11.
In the verb. finit.: Quidam prius tuto sale sex horis (ova) adoperiunt, Col. 8, 6: pellem setis adoperuit, Lact. Op. Dei, 7.
Hence, ădŏpertē, adv., v. the foll. art.

ădŏpertē, adv. [adoperio], covertly, in a dark, mysterious manner: denuntiare, Mart. Cap. 8, p. 303.