Lewis & Short

Symmăchus, i, m., Q. Aurelius, a consul and prefect of Rome towards the end of the fourth century, distinguished as an orator and author of Epistulae in ten books, still extant; cf. Macr. S. 5, 1, § 7; Sid. Ep. 1, 1; Amm. 21, 12, 24; Aus. Ep. 17.
Hence, Symmăchĭānus, a, um, adj., of or belonging to Symmachus: illud dictum, Sid. Ep. 8, 10.