1. In a manner that suggests something important or bad is about to happen; ominously
The sky darkened portentously before the storm arrived.
O céu escureceu portentosamente antes da tempestade chegar.
2. In a solemn, impressive, or self-importantly serious manner
He spoke portentously about the importance of the decision.
Ele falou portentosamente sobre a importância da decisão.