Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
imperco (inp-, also imparco), ĕre, v. n., to spare (Plautin.): huic impercito, Plaut. Cas. 4, 4, 12: imperce, quaeso, spare yourself, id. Am. 1, 3, 2.