Definition of "portentous"

Well may it sort [be fitting] that this portentous figure comes armed through our watch, so like the King that was and is the question of these wars.

c. 1599-1602, William Shakespeare, The Tragedy of Hamlet, Prince of Denmark, act 1, scene 1; republished as Hamlet, New York: Dover Publications, Inc., 1992, page 4

It is no longer in my possession, but my memory holds almost every word of its portentous message; and again I affirm my confidence in the sanity of the man who wrote it.

1931, H. P. Lovecraft, chapter 2, in The Whisperer in Darkness

The chaplain's first mention of the name Yossarian! had tolled deep in his memory like a portentous gong.

1961 November 10, Joseph Heller, “Chief White Halfoat”, in Catch-22 […], New York, N.Y.: Simon and Schuster, page page

If the siege of Boston seemed to end in anticlimax, with the sudden retreat of British military forces, the siege of New York City seen from the inside, from under siege, seemed infinitely more portentous: “The Day is Come that in all Probility on which Depends the Salvation of this Countery.”

2009, Konstantin Dierks, “Revolution and War”, in In My Power: Letter Writing and Communications in Early America, Philadelphia, Pa.: University of Pennsylvania Press, page 225

The main problem with To Believe, however, is that by combining two of their previous styles, the Cinematic Orchestra have landed on a sound that has become ubiquitous in the years they’ve been away – portentous, restrained and impeccably tasteful electronica.

2019 March 15, Rachel Aroesti, “The Cinematic Orchestra: To Believe review – soundscape originators' accomplished return”, in The Guardian‎