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®

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