Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
impromptus (inpr-), a, um. adj. [2. in-promptus], not ready, not quick (rare): infacundior et lingua impromptus, Liv. 7, 4, 5: imprompto jam Arminio ob continua pericla, Tac. A. 2, 21: sermone impromptus Latio, Aus. Idyll. 2, 9.