Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
per-dĭu, adv., for a great while, very long (rare but class.), Varr. R. R. 1, 58: perdiu nihil eram auditurus, Cic. Att. 3, 22, 4; id. de Or. 1, 2, 8.