1723, from assumed L. pl. Ovates, from Gk. Ouateis "soothsayers, prophets," mentioned by Strabo as a third order in the Gaulish hierarchy, from P.Celt. *vateis, pl. of *vatis, cognate with L. vatis, O.Ir. faith, Welsh ofydd. The modern word, and the artificial senses attached to it, are from the 18c.
Celtic revival and the word appears first in Henry Rowlands.