Lewis & Short

praeostendo, endi, ensum, 3, v. a., to show beforehand, display before, make clear (late. Lat.): hoc monstrare debent, hoc ostendere vel praeostendere, Aug. Civ. Dei, 21, 8, 6.
Absol.: portenta a portendendo, id est, praeostendendo, Aug. Civ. Dei, 21, 8, 6; cf. Isid. 11, 3, 3.
Hence, praeosten-sus, a, um, Part., foreshown (eccl. Lat.): sabbatum praeostensum et praedictum, Tert. adv. Jud. 4: sacrificia, id. ib. 5.