Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
per-trĕpĭdus, a, um, adj., in great alarm or in great haste: senatus pertrepidus in aedem Concordiae concurrit, Capitol. Maxim. et Balb. 1, 1 dub. (Peter, praetrepidus).