Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
furnus (fornus, Varr. ap. Non. 531, 32), i, m. [Sanscr. ghar-, to lighten; cf.: fornus, fornax, and v. formus], an oven (syn.: caminus, fornax, clibanus): in furnum calidum condito, Plaut. Cas. 2, 5, 1: malim istius modi mi amicos furno mersos quam foro, id. Ep. 1, 2, 16; Varr. l. l.; Plin. 13, 4, 9, § 48; 19, 1, 3, § 18; 20, 9, 39, § 99; Ov. F. 6, 313. Used by the Romans as a warming-place, Hor. S. 1, 4, 36; id. Ep. 1, 11, 13.