Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
oppando, pandi, pansum, or passum, 3, v. a. [ob-pando], to spread or stretch out against or before, to spread out (post-class.): aliquid ad flatus helices, Grat. Cyn. 55: cornibus oppansis et summā fronte coruscum (of the cross of Christ), Prud. Psych. 410: aulaei vice oppansā, Tert. Apol. 48 fin.; Hier. Ep. ad Galat. 1, v. 11, 12; Vulg. Exod. 35, 12.
Hence, oppansum (-passum), i, n., a covering, envelope (eccl. Lat.): corporis, Tert. Anim. 53.