Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
cantābundus, a, um, adj. [canto], singing (rare: sedeo ego cantabundus et stellas numero, Petr. 62, 4: Gallus suā disciplinā scuto projecto cantabundus (constitit), Quadrig. ap. Gell. 9, 13, 16 dub. (Hertz, cunctabundus).