Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
ap-pugno (adp-, Halm), āre, v. a., to fight against, attack, assault (only in Tac. in the three foll. exs.): castra adpugnare, Tac. A. 4, 48: castellum, id. ib. 15, 13: classem, id. ib. 2, 81.