Lewis & Short

sexāgĭes or sexāgĭens (collat. form sexāgēsĭes, Mart. Cap. 6, § 610), num. adv. [id.], sixty times: sestertium sexagies, i. e. sixty times a hundred thousand, six millions of sesterces (v. sestertius), Caes. B. C. 1, 23; Cic. Phil. 2, 18, 45; and, in the same sense, simply sexagies, id. Rosc. Am. 2, 6.