Definition of "portentously"

The cold light of the moon touched every face with unnatural paleness; and the silence was unbroken and portentously profound.

1834, L[etitia] E[lizabeth] L[andon], chapter XXXVIII, in Francesca Carrara. […], volume III, London: Richard Bentley, […], (successor to Henry Colburn), page 318

At that moment a long, scraggy individual in a checked suit poked his head into the bar, looked around portentously, whistled mysteriously to my informant, and jerked his thumb and head in the direction of the door.

1946, George Johnston, Skyscrapers in the Mist, page 121

Shirky ends the story of the lost Sidekick by asking, portentously, “What happens next?”—no doubt imagining future waves of digital protesters.

2010 September 27, Malcolm Gladwell, “Small Change”, in The New Yorker‎