Nonus, Adiectinum numerale. Plin. The ninth.Non nisi nonis mensibus redeunt. Plin. They returne but uery ninth moneth.
Lewis and Short: Latin dictionary
nōnus, a, um, adj. ord. [for novenus, from novem], the ninth: terra nona, Cic. Rep. 6, 18, 18: accedes opera agro nona Sabino, Hor. S. 2, 7, 118.—II.Subst.: nōna, ae, f.A. (Sc. hora.) The ninth hour of the day, i. e. the third before sunset, at which hour business was ended at Rome: post nonam venies, Hor. Ep. 1, 7, 71; Mart. 4, 8, 5.—B. (Sc. pars.) The ninth part: nonas praedae vovere, Just. 20, 3, 3.—Hence, adv.: nōnō, ninthly, Cassiod. de Anim. 12.