Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
sittybus, i, m., a strip of parchment, attached to a roll or book, bearing the title and the author’s name, Cic. Att. 4, 5, 3; 4, 8, a, 2 B. and K. (in some editions sillybus; cf. σιλλύβους, id. ib. 4, 4, b, 1).