Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Sōpăter, tri, m., = Σώπατρος, a Greek proper name, e. g. a victim of Verres: Halicyensis, Cic. Verr. 2, 2, 28, § 68.
Another, a witness against Verres, Cic. Verr. 2, 4, 39, § 85.