Lewis & Short

rōbur (rōbor, v. Lucr. p. 140 Lachm.; also an older form rōbus, Cato, R. R. 17, 1; Col. 2, 6, 1; cf. Paul. ex Fest. p. 264 Müll.), ŏris, n. [cf. Sanscr. radh-as, abundance; Gr. ῤώννυμι for ῤώθνυμι, to strengthen, ῤώμη], a very hard kind of oak (cf.: quercus, ilex), Plin. 16, 6, 8, § 19; 16, 7, 10, § 28; 16, 38, 73, § 186; 16, 40, 76, § 204; 16, 40, 77, § 218.
Hence,

  1. I. Lit.
      1. 1. In gen., a very hard kind of tree or wood: morsus roboris, i. e. of the wild olive, Verg. A. 12, 783 (a little before: foliis oleaster amaris Hic steterat); so of the same, id. G. 2, 305; cf.: solido de robore myrtus, id. ib. 2, 64: annoso validam robore quercum, i. e. of an old and sturdy trunk, id. A. 4, 441; so, annoso robore quercus, Ov. M. 8, 743: antiquo robore quercus, with ancient trunk, Verg. G. 3, 332: Massyla, i. e. citri, Stat. S. 3, 3, 94; also, Maurorum, id. ib. 4, 2, 39.
      2. 2. Absol., usu., an oak-tree, an oak in gen.: fixa est pariter cum robore cervix, i. e. was pinned fast to the oak, Ov. M. 3, 92: agitata robora pulsant (delphines), id. ib. 1, 303.
      3. 3. Oak-wood, oak: naves totae factae ex robore, Caes. B. G. 3, 13; cf.: (sapiens) non est e saxo sculptus aut e robore dolatus, Cic. Ac. 2, 31, 101; and with this cf. id. Div. 2, 41, 85.
        Poet.: illi robur et aes triplex Circa pectus erat, Hor. C. 1, 3, 9; cf.: o saxis nimirum et robore nati! Stat. Th. 4, 340.
  2. II. Transf.
    1. A. Of things made of oak or of any other hard wood.
      1. 1. In gen.: Lacedaemonii cottidianis epulis in robore accumbunt, i. e. on oaken, hard benches, Cic. Mur. 35, 74.
        So of the wooden horse before Troy: sacrum, Verg. A. 2, 230; of a lance: ferro praefixum, id. ib. 10, 479; Sil. 2, 244; 267; of a club, Ov. M. 12, 349; Mart. 9, 44, 4 et saep.: aratri, i. e. the oaken plough, Verg. G. 1, 162; Val. Fl. 7, 555.
      2. 2. In partic., the lower and stronger part of the prison at Rome, built by Servius Tullius, was called Robur (also Tullianum): Robus in carcere dicitur is locus, quo praecipitatur maleficorum genus, quod ante arcis robusteis includebatur, Paul. ex Fest. s. v. robum, p. 264 Müll.: in robore et tenebris exspiret, Liv. 38, 59 fin.: robur et saxum minitari, Tac. A. 4, 29; Val. Max. 6, 3, 1: verbera, carnifices, robur, Lucr. 3, 1017; Hor. C. 2, 13, 19 (v. carcer and Tullianum).
    2. B. Hardness, strength, firmness, vigor, power (cf. vires; v. Fabri ad Liv. 21, 1, 2).
      1. 1. Lit.: duri robora ferri, Lucr. 2, 449; so, ferri, Verg. A. 7, 609: saxi, Lucr. 1, 882: navium, Liv. 37, 30: omnia pariter crescunt et robora sumunt, gain strength, Lucr. 5, 820; 895; cf.: qui si jam satis aetatis atque roboris haberet, ipse pro Sex. Roscio diceret, Cic. Rosc. Am. 51, 149: paululum jam roboris accessit aetati, id. Cael. 30, 73: solidaeque suo stant robore vires, Verg. A. 2, 639; Vulg. Judic. 8, 21: si quod est robur, Flor. 2, 1, 1.
      2. 2. Trop., power, strength, force, vigor (very freq.): alter virtutis robore firmior quam aetatis, Cic. Phil. 10, 8, 16: in animi excelsi atque invicti magnitudine ac robore, id. Off. 1, 5, 14; so, animi (with magnitudo), id. de Or. 2, 84, 343; id. Tusc. 1, 40, 95: robur incredibile animi, id. Mil. 37, 101: quantum in cujusque animo roboris est ac nervorum, id. Fam. 6, 1, 3: multo plus firmamenti ac roboris, id. Imp. Pomp. 4, 10; so (with firmamentum) id. Mur. 28, 58; (with firmitas) id. Fin. 5, 5, 12: hi tot equites Romani quid roboris hujus petitioni attulerunt? id. Planc. 8, 21: pectus robore fultum, Ov. Tr. 5, 12, 11: te mea robora fallunt, id. H. 16, 367: velocitate pari, robore animi virumque praestanti, Liv. 24, 26, 11: verba quanti roboris plena, Sen. Ep 10, 3: qui robur aliquod in stilo fecerint, Quint. 10, 3, 10; cf.: robur oratorium adicere sententiis, id. 10, 5, 4; 8, prooem. § 3: illi robur et aes triplex Circa pectus erat, Hor. C. 1, 3, 9; cf.: O saxis nimirum et robore nati, Stat. Th. 4, 340.
        1. b. Authority: nostrarum constitutionum, Just. Inst. prooem. 6.
        2. c. Concr., the strongest, most effective, or best part, the pith, kernel, strength of any thing; of soldiers, the flower of the troops, choice troops, etc. (freq. and class.): versaris in optimorum civium vel flore vel robore, Cic. Or. 10, 34: et robur et suboles militum interiit, Asin. ap. Cic. Fam. 10, 33; cf.: quod fuit roboris, duobus proeliis interiit, Caes. B. C. 3, 87: quod roboris ea provincia habuerat, Liv. 30, 2; Ov. M. 14, 454 al.: senatūs robur, Liv. 5, 39.
          Plur.: tunc C. Flavius Pusio, Cn. Titinnius, C. Maecenas, illa robora populi Romani, Cic. Clu. 56, 163: haec sunt nostra robora, id. Att. 6, 5, 3; Liv. 7, 7; 12; 21, 54; 22, 6; 23, 16; 25, 6 init.: robora pubis, Verg. A. 8, 518; Ov. M. 7, 510: ingentia robora virorum, Plin. Pan. 34, 3: conferta robora virorum, Curt. 3, 5, 13: betae, i. e. stalks, Col. poët. 10, 326.
          Of a place, a stronghold: quod coloniam virium et opum validam robur ac sedem bello legisset, Tac. H. 2, 19.
          Absol.: robus, the name of an excellent kind of wheat: quoniam et pondere et nitore praestat, Col. 2, 6, 1.