Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
pĕr-imbĕcillus, a, um, adj., very weak or feeble: collum anserum, Varr. R. R. 3, 10, 5: quod quidem est natum perimbecillum est, Cic. Att. 10, 18, 1: verbum, Sulp. Sev. Chron. 2, 17, 3.