Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
fĭmus, i (also fĭmum, i, n., Plin. 28, 17, 70, § 234 sq.; 29, 5, 32, § 101; 30, 9, 23, § 76; Lact. Opif. D. 11, 20: fimo, abl. fem., Apul. Met. 7, p. 200 fin.), m. [Sanscr. dhūmas, smoke; dhū-lis, dust; cf. Gr. θυ-, θύνω, θῦμα, θύος; Lat. sub-fīo, sub-fimen; Germ. Dunst; Engl. dust], that which fertilizes or manures, dung, ordure, excrement (only in the sing., Diom. p. 314 P.; for syn. cf.: stercus, merda, quisquiliae).