Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
cūlus, i, m. [perh. kindr. with κοῖλος, of a curving form; cf. 1. anus], the posteriors, fundament, Cat. 23, 19; 33, 4; 97, 2; Mart. 3, 98, 1.