Source : Webster's Revised Unabridged Dictionary (1913)

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

            For, I believe, they are portentous things. --Shak.

            Victories of strange and almost portentous splendor.
                                                  --Macaulay.

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

Source : WordNet®

portentous
     adj 1: of momentous or ominous significance; "such a
            portentous...monster raised all my curiosity"- Herman
            Melville; "a prodigious vision" [syn: {prodigious}]
     2: of ominous significance [syn: {fateful}, {foreboding(a)}]
     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}]