Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Perseus.
Ăărōn (Ăārōn, Prud. Psych. 884), indecl. or ōnis, m., [??] Aaron, brother of Moses, and first high-priest of the Hebrews, Vulg. Exod. 4, 14; 6, 25 al.