Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
suppus, a, um, adj. (of uncertain signif.; acc. to Fest., = supinus): suppum antiqui dicebant, quem nunc supinum dicimus, ex Graeco, videlicet pro aspiratione ponentes S litteram … Ejus vocabuli meminit etiam Lucilius: si vero das, quod rogat, et si suggeri suppus, Fest. p. 290 Müll.: (jactum) unionem canem, trinionem suppum, quaternionem planum vocabant, Isid. Orig. 65: vagari animalia suppa, walk inverted, head downwards, Lucr. 1, 1061.