Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
imprŏpĕrĭum (inpr-), ĭi, n. [2. impropero], a reproach, taunt (eccl. Lat.), Lact. 4, 18, 32; Vulg. Rom. 15, 3 al.; cf. improperium ὀνείδισις, Gloss. Philox.