Foster-brother - Vine's Expository Dictionary of New Testament Words
Foster-brother
[ 1,,G4939, suntrophos ]primarily denotes one nourished or brought up with another" (sun, "with," trepho, "to rear"); it is rendered "foster-brother" in Acts 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.