Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Bonna, ae, f., a city on the Rhine, now Bonn, Tac. H. 4, 19; 4, 25; 70, 77; 5, 22; Flor. 4, 12, 26.
Hence, Bonnensis, e, adj., of Bonna: castra, Tac. H. 4, 20; 4, 62: proelium, id. ib. 4, 20.