Portentous means ominous, or self-important in manner. A portentous silence, portentous prose, portentous warnings.
The two senses pull apart, since one means foreboding and the other means pompous, and only the context tells you which. Careful writers use pretentious for the second, which keeps portentous for omens.
1adjectiveportentousOminous, or self-important in manner.
“Dark clouds gathered in a portentous sky before the storm broke.”
portentous is a quality.
Latin portentosus comes from portentum, an omen, from portendere, to foretell.
Proto-Indo-European
*per-
English, today
portentous
Of multiple origins:
Etymology from Wiktionary, CC BY-SA