Lewis & Short

spĕcus, ūs (abl. plur. specibus, Sen. Cons. ap. Front. Aquaed. 125; on the various forms found only in the grammarians, v. Neue, Formenl. 1, 569 sq.), m. (f. and n. v. infra) [perh. root spec-; v. specio, and so orig. a hole, aperture; but cf. σπέος].

  1. I. Lit., a cave, cavern, grot, den; a cavity, chasm, natural or artificial; of the latter kind, a ditch, drain, canal, channel, covered water-course, a pit in mines, etc. (cf. spelunca).
          1. (α) Masc. (class. and freq.): inferum vastos specus, Enn. ap. Non. 222, 32 (Trag. v. 218 Vahl.): specus tenebricosus, Varr. ib. 222, 31: fons ex opaco specu, Liv. 1, 21; so abl. sing., id. 1, 56, 10; 10, 10, 1: forum medium ferme specu vasto collapsum dicitur, id. 7, 6; so sing., Ov. M. 3, 29; 7, 409; 11, 235; id. F. 4, 495; Liv. 10, 1, 5; Tac. A. 16, 1, 3; 16, 4, 59.
            Plur.: quos agor in specus? Hor. C. 3, 25, 2: paucos specus in extremo fundo, et eos quidem subterraneos, * Cic. Att. 15, 26, 4; so of drains, ditches, Varr. R. R. 3, 17 fin.; Cat. 61, 28; Liv. 39, 13, 13; Tac. A. 12, 57; id. G. 16; Sen. Med. 741 al.
          2. (β) Fem. (ante- and post-class.): concava specus, Enn. ap. Non. 222, 23 (Ann. v. 420 Vahl.); cf. Serv. Verg. A. 7, 568; Pac. ap. Non. 223, 2 Müll. (Trag. p. 91 Rib.); id. ap. Fest. p. 343 Müll. (l. l. p. 73 Rib.): specum quandam nactus remotam latebrosamque, in eam me penetro et recondo, Gell. 5, 14, 18; Sil. 6, 276.
          3. (γ) Neutr. (anteclass. and poet.): hic specus horrendum, Verg. A. 7, 568; Serv. ad loc.: invisum caelo specus, Sil. 13, 425; Ps.-Quint. Decl. 10, 19.
          4. (δ) Acc. to the second declension: speca apposita, Cato ap. Prisc. p. 713 P.: altis claudere specis, Att. ap. Non. 487, 25 (Fragm. Trag. v. 63 Rib.).
  2. II. Transf., a hollow, cavity of any kind (poet.): specus atri vulneris, Verg. A. 9, 700 Serv.: quos Capacis alvi mersit tartareo specu, Phaedr. 4, 6, 10; of a snake’s belly, Sil. 6, 276; cf. of the belly of the Trojan horse, Petr. poët. 89, 2, 7.
    In mal. part., Auct. Priap. 83, 34.