Lewis & Short

arduus, a, um, adj. [akin to ΑΡΔΩ, ἄρσω = to water, to cherish; ἀλδαίνω = to make grow; ἀλδήεις = growing; alo, altus, q. v.; 1. ad-oleo, ad-olesco; related to arbor, arbutus as ἐρυθρός, Germ. roth, Engl. red, is related to ruber; Ardea was perh. so called from its lofty situation; cf. Arduenna], high, elevated, lofty, steep (syn.: altus, celsus, sublimis).

  1. I. Lit.: Pergama ardua, Enn. ap. Macr. S. 6, 2: aether, Ov. M. 1, 151: sidera, id. ib. 1, 730: cedrus, id. Am. 1, 14, 12: cervix equi, Hor. S. 1, 2, 89: et campo sese arduus infert (Turnus), Verg. A. 9, 53.
    Also in prose in Gell.: supercilia, i. e. proudly elevated, Gell. 4, 1, 1: confragosus atque arduus clivis, steep, Varr. R. R. 1, 18, 4: ascensus, Cic. Verr. 2, 4, 23: arduus ac difficilis ascensus, Liv. 25, 13: ardua et aspera et confragosa via, id. 44. 3: via alta atque ardua, Enn. ap. Cic. Tusc. 1, 16, 37: mons, Ov. M. 1, 316: Tmolus, id. ib. 11, 150 al.
    Hence, subst.: arduum, i, n., a steep place, a steep: Ardua dum metuunt, amittunt vera viaï, Lucr. 1, 659: in ardua montis Ite, Ov. M. 8, 692: ardua terrarum, Verg. A. 5, 695: per arduum scandere, Hor. C. 2, 19, 21: in arduo, Tac. A. 2, 47: in arduis ponet nidum suum, Vulg. Job, 39, 27: ardua Alpium, Tac. H. 4, 70: castellorum, id. A. 11, 9: ingressi sunt ardua, Vulg. Jer. 4, 29.
  2. II. Trop.
    1. A. That is difficult to reach or attain, difficult, laborious, hard, arduous: magnum opus omnino et arduum conamur, Cic. Or. 10, 33: rerum arduarum ac difficilium perpessio, id. Inv. 2, 54; so id. Leg. 1, 13: id arduum factu erat, Liv. 8, 16; Tac. A. 4, 4: victoria, Ov. M. 14, 453: virtus, Hor. C. 3, 24, 44: nil mortalibus arduum est, id. ib. 1, 3, 37.
      Subst.: nec fuit in arduo societas, Tac. A. 12, 15.
    2. B. Troublesome, unpleasant: in primis arduum videtur res gestas scribere, Sall. C. 3, 2, upon which Gellius remarks: Arduum Sallustius non pro difficili tantum, sed pro eo quoque ponit, quod Graeci χαλεπὸν appellant: quod est cum difficile tum molestum quoque et incommodum et intractabile, Gell. 4, 15: quam arduum onus, Tac. A. 1, 11.
    3. C. Of fortune, difficult, adverse, inauspicious: aequam memento rebus in arduis Servare mentem, in adversity, Hor. C. 2, 3, 1.
      Note: Comp. arduior: iter longius arduiusque erat, Cato ap. Prisc. p. 600 P.
      Sup. arduissimus: asperrimo atque arduissimo aditu, Cato ap. Prisc. p. 600 P.; cf.: assiduus, egregius, industrius, perpetuus, and Rudd. I. p. 180, n. 58.
      Adv. not used.