Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
mīrābundus, a, um, adj. [miror], wondering, astonished, full of wonder or astonishment (mostly Livian and post-class.): nova res mirabundam plebem convertit, quidnam incidisset, cur, etc., Liv. 3, 38: Poeni mirabundi, unde, etc., id. 25, 37, 12; Curt. 9, 9, 26.
With acc.: mirabundi bestiam, App. M. 4, p. 150, 4.