Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
† 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.