Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
a-stŭpĕo (ads-, Merk.), ēre, v. n., to be amazed at or on account of, to be astonished at (rare, and mostly poet.; perh. not before the Aug. per.): Adstupet ipse sibi, Ov. M. 3, 418: Cui fida manus proceresque socerque Adstupet oranti, Stat. Th. 3, 406: divitiis, Sen. Tranq. Vit. 1, 8; Sid. Ep. 5, 5.
Of inanimate things: nemus adstupet, Stat. Th. 2, 13.