Foster-brother
New TestamentVine's Expository Dictionary of Biblical Words
Definition
1suntrophosG4939
primarily denotes "one nourished or brought up with another" (sun, "with," trepho, "to rear"); it is rendered "foster-brother" in Act 13:1, RV. It has, however, been found in Hellenistic usage as a court term, signifying an intimate friend of a king (Deissmann), and this would seem to be the meaning regarding Manaen and Herod the Tetrarch.