Lewis & Short

ăthĕŏs (-us), i, m., = ἄθεος, he that does not believe in God, an atheist: Diagoras, atheos qui dictus est, Cic. N. D. 1, 23, 63 B. and K. (Orelli writes it as Greek); so Arn. 3, p. 116; 5, p. 178.