Lewis & Short

1. dĕcĭmus or dĕcŭmus (the latter form prevailed in the later law lang.; hence, decumanus), a, um, adj. [decem with superl. ending], the tenth.

  1. I. Prop.: mensis, Plaut. Am. 1, 2, 19; cf. Ter. Ad. 3, 4, 29: legio, Caes. B. G. 1, 40; cf. ib. 41; 42 al.: decima hora, Cic. Phil. 2, 31; and without hora, Auct. Her. 4, 51: annus, Verg. A. 9, 155: septuma (dies) post decumam, i. e. the seventeenth, id. G. 1, 284 Voss.: cum decumo efficit ager, i. e. tenfold, Cic. Verr. 2, 3, 47, § 112; so, extulisset, ib. § 113.
        1. * b. dĕcĭmum, adv. (like tertium, quartum, etc.; cf. Gell. 10, 1), for the tenth time, Liv. 6, 40.
    1. B. Subst.: dĕcĭma (dĕcŭma), ae, f. (sc. pars), the tenth part, tithe.
      1. 1. As an offering: testatur Terentius Varro … majores solitos decimam Herculi vovere, Macr. S. 3, 12; so Varr. L. L. 6, § 54 Müll.; Just. 18, 7, 7; cf. with pars; Naev. ap. Prisc. p. 874 P.; Plaut. Stich. 1, 3, 80: tibi (sc. Pythico Apollini) hinc decumam partem praedae voveo, Liv. 5, 21; cf.: cum vovissent Apollini decumas praedae, Just. 20, 3, 3; cf. id. 18, 7, 7; Vulg. Gen. 14, 20; so esp. of the tithes given by the Hebrews to support the priesthood, id. Num. 18, 21 et saep.
      2. 2. A largess openly bestowed by public men on the people: Oresti nuper prandia in semitis decumae nomine magno honori fuerunt, Cic. Off. 2, 17, 58; so Suet. Calig. 26; id. Galb. 15; Tac. H. 1, 20.
      3. 3. A tithe, as a tax on landholders in the provinces, Cic. Verr. 2, 3, 10, § 25; more freq. in plur., id. ib. 2, 3, 39, § 89 sq.
      4. 4. A tithe, as conveyed by last will: decimas uxoribus dari, Trach. ap. Quint. 8, 5, 19.
  2. II. Meton. (like decem, decies, etc.), considerable, large, immense (poet.): vastius insurgens decimae ruit impetus undae, Ov. M. 11, 530 (cf.: decimanus, no. II., and in Gr. τρικυμία); so of billows, Sil. 14, 122; Luc. 5, 672; Val. Fl. 2, 54 (decimus by circumlocut.: qui venit hic fluctus, fluctus supereminet omnes; posterior nono est undecimoque prior, Ov. Tr. 1, 2, 50).