Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Mŏdestīnus, i, m., a Roman surname, Inscr. Marin. Frat. Arv. 36; Inscr. Grut. 712, 2.
So, Herennius Modestinus, a celebrated lawyer under Alexander Severus, Lampr. Alex. Sev. 68.