Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
The word synonymus could not be parsed. Trying a normal dictionary lookup:
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synōnymum, i, n., = συνώνυμον, a word having the same meaning with another, a synonym, Front. Eloqu. p. 237; Prisc. 579 P.; Serv. Verg. A. 2, 128. († synŏphītes, ae, m., a read. in Plin. 37, 10, 59, § 162 for synnephitis.)