The Collaborative International Dictionary of English v.0.48:

Portentous \Por*tent"ous\, a. [L. portentosus.]
   1. Of the nature of a portent; containing portents;
      foreshadowing, esp. foreshadowing ill; ominous.
      [1913 Webster]

            For, I believe, they are portentous things. --Shak.
      [1913 Webster]

            Victories of strange and almost portentous splendor.
                                                  --Macaulay.
      [1913 Webster]

   2. Hence: Monstrous; prodigious; wonderful; dreadful; as, a
      beast of portentous size. --Roscommon.
      [1913 Webster] -- Por*tent"ous*ly, adv. --
      Por*tent"ous*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

portentous
    adj 1: of momentous or ominous significance; "such a
           portentous...monster raised all my curiosity"- Herman
           Melville; "a prodigious vision" [syn: portentous,
           prodigious]
    2: ominously prophetic [syn: fateful, foreboding(a),
       portentous]
    3: puffed up with vanity; "a grandiloquent and boastful manner";
       "overblown oratory"; "a pompous speech"; "pseudo-scientific
       gobbledygook and pontifical hooey"- Newsweek [syn:
       grandiloquent, overblown, pompous, pontifical,
       portentous]

Moby Thesaurus II by Grady Ward, 1.0:

86 Moby Thesaurus words for "portentous":
   alarming, amazing, apocalyptic, astonishing, astounding,
   awe-inspiring, awesome, bad, baleful, baneful, black, bodeful,
   boding, dark, dire, doomful, dreary, earnest, evil, evil-starred,
   exceptional, extraordinary, fabulous, fantastic, fatal, fateful,
   foreboding, forerunning, forewarning, formidable, gloomy, grave,
   heavy, ill, ill-boding, ill-fated, ill-omened, ill-starred,
   imposing, inauspicious, indescribable, ineffable, legendary,
   lowering, marvelous, menacing, mind-boggling, miraculous,
   momentous, mythical, noteworthy, of evil portent, ominous,
   phenomenal, portending, precautional, precautionary, precursive,
   precursory, premonitory, prodigious, prodromal, prodromic,
   remarkable, serious, sinister, sober, solemn, somber, star-crossed,
   stupefying, stupendous, threatening, unexampled, unfavorable,
   unfortunate, unlucky, unparalleled, unprecedented, unpromising,
   unpropitious, unspeakable, untoward, weighty, wonderful,
   wondrous