Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Thersītes, ae, m., = Θερσίτης, a Greek before Troy, famous for his ugliness and scurrility, Ov. M. 13, 233; id. P. 3, 9, 10; Juv. 11, 31.
Hence, a contemptible person, Juv. 8, 269; and for a calumniator, reviler, Sen. Ira, 3, 23, 3.