Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
aggressor (adg-), ōris, m. [aggredior], one that attacks, an assailant, aggressor (only in the Lat. of the Pandects), Dig. 29, 5, 1 fin.; also for a robber, ib. 48, 9, 7 al.