Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
faenĭ-sex (fēn-, foen-), ĕcis (faenĭ-sĕca, ae, Pers. 6, 40), m. [faenum, I.; seco].
fēnĭsex, v. faenisex.