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

portentously
    adv 1: in a portentous manner; "portentously, the engines began
           to roll"