Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
per-cŭpĭo, pĕre, v. a., to wish greatly, desire earnestly, to long (ante-class.): percupio obsequi gnato meo, Plaut. As. 1, 1, 61; Ter. Eun. 5, 2, 57.