IPA: /pɔːrˈtɛntəsli/
KK: /pɔrˈtɛntəsli/
In a way that suggests something important or serious is going to happen, often with a sense of warning or foreboding.
The sky darkened portentously, signaling an approaching storm.
Portentously → It is formed from "portent" (from Latin "portentum", meaning a sign or omen) and the suffix "-ously" (meaning in a manner characterized by). The word "portentously" means in a manner that is ominous or foreboding, as if indicating something significant or threatening is about to happen.
Think of a 'sign' or 'omen' ('portent') that is presented in a way that is 'characteristic' ('-ously') of something serious or foreboding.