1 (literary) important as a sign or a warning of sth that is going to happen in the future, especially when it is sth unpleasant:
a portentous dream / sign
2 (formal, disapproving) very serious and intended to impress people:
a portentous remark / voice
portentously adverb:
She frowned portentously to add weight to her suggestion.
portentousness noun [U]
