The word portentous appears in the following sentences from Moby-Dick:
Chapter 1 > Paragraph 14 > Sentence 2:
Such a portentous and mysterious monster roused all my curiosity.
Chapter 3 > Paragraph 2 > Sentence 1:
But what most puzzled and confounded you was a long, limber, portentous, black mass of something hovering in the centre of the picture over three blue, dim, perpendicular lines floating in a nameless yeast.
Chapter 3 > Paragraph 2 > Sentence 5:
But last all these fancies yielded to that one portentous something in the picture's midst.
Chapter 34 > Paragraph 8 > Sentence 4:
Such portentous appetites had Queequeg and Tashtego, that to fill out the vacancies made by the previous repast, often the pale Dough-Boy was fain to bring on a great baron of salt-junk, seemingly quarried out of the solid ox.
Chapter 42 > Paragraph 25 > Sentence 1:
But not yet have we solved the incantation of this whiteness, and learned why it appeals with such power to the soul; and more strange and far more portentous—why, as we have seen, it is at once the most meaning symbol of spiritual things, nay, the very veil of the Christian's Deity; and yet should be as it is, the intensifying agent in things the most appalling to mankind.
Chapter 74 > Paragraph 12 > Sentence 1:
But come out now, and look at this portentous lower jaw, which seems like the long narrow lid of an immense snuff-box, with the hinge at one end, instead of one side.
Concordance for the word portentous from Moby-Dick.