Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
rīdĭbundus, a, um, adj. [rideo], laughing (ante-class.), Plaut. Ep. 3, 3, 32; cf. Gell. 11, 15, 2 sq.; Cato, Orat. 1, Fragm. 12: ridibunda ea dicitur quae ridentem agit aut simulat, Ter. Sc. ap. Gell. 11, 15, 3.