The Collaborative International Dictionary of English v.0.48:

Mountainous \Moun"tain*ous\, a. [F. montagneux, L. montaniosus.]
   1. Full of, or containing, mountains; as, the mountainous
      country of the Swiss.
      [1913 Webster]

   2. Inhabiting mountains. [Obs.] --Bacon.
      [1913 Webster]

   3. Large as, or resembling, a mountain; huge; of great bulk;
      as, a mountainous heap. --Prior.
      [1913 Webster]

WordNet (r) 3.0 (2006):

mountainous
    adj 1: having hills and crags; "hilly terrain" [syn: cragged,
           craggy, hilly, mountainous]
    2: like a mountain in size and impressiveness; "mountainous
       waves"; "a mountainous dark man"
    3: containing many mountains

Moby Thesaurus II by Grady Ward, 1.0:

66 Moby Thesaurus words for "mountainous":
   Atlantean, Brobdingnagian, Cyclopean, Gargantuan, Herculean,
   Homeric, abysmal, alpen, alpestrine, alpigene, alpine,
   amplitudinous, astronomic, astronomical, awesome, boundless, bulky,
   colossal, cosmic, craggy, elephantine, enormous, epic, extensive,
   formidable, galactic, giant, giantlike, gigantic, heroic, high,
   hilly, huge, immeasurable, immense, infinite, jumbo, king-size,
   knobby, large, mammoth, massive, massy, mighty, monster, monstrous,
   monticuline, monumental, mountained, outsize, overgrown,
   prodigious, profound, rolling, sizable, spacious, staggering,
   steep, stupendous, subalpine, titanic, towering, tremendous, vast,
   voluminous, weighty