Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
sextō, adv., v. 1. sextus, B. 2.
1. sextus, a, um, num. ord. adj. [sex], the sixth, Plaut. Ps. 4, 2, 5: sextus ab urbe lapis, Ov. F. 2, 682: sextus decimus ab Hercule, Vell. 1, 6, 5: hic annus sextus, postquam ei rei operam damus, Plaut. Men. 2, 1, 9; id. Most. 4, 2, 41: sexto decimo anno, Cic. Rep. 2, 33, 57: sextus locus est, etc., id. Inv. 1, 53, 102: sextus decimus (locus), id. ib. 1, 56, 109; Tac. A. 1, 17: sexta decima legio, id. ib. 1, 37 al.: sexta decima (sc. hora), Mart. Cap. 6, § 696; for which also, in one word: post sextumdecimum annum, the sixteenth, Liv. 30, 19: abdicat die sextodecimo, id. 4, 34: sextodecimo Calendas Jan., Col. 11, 2, 94.
In gram.: sextus casus, the ablative case, Quint. 1, 4, 26.