Lewis & Short

ethnĭcus, a, um, adj., = ἐθνικός, heathenish, pagan; and subst.: ethni-cus, i, m., a heathen, Tert. Pud. 9; Vulg. Matt. 5, 47; 18, 17.
Adv.: ethnĭce, heathenishly: vivere, Tert. Pud. 9.