In a manner that is ominous or foreboding, suggesting that something bad is going to happen. The sky darkened portentously, hinting at the approaching storm.
In a way that is pompous or self-important, often to impress others. He spoke portentously about his achievements, trying to impress the audience.
To a remarkable or prodigious degree; impressively or astonishingly. The cliffs rose portentously above the crashing waves.