Definition of "ominously"

From first to last he was ominously polite, and ominously silent.

1868 January 4 – June 6, [William] Wilkie Collins, “Second Period. The Discovery of the Truth. (1848–1849.) […] [Fourth Narrative. Extracted from the Journal of Ezra Jennings.]”, in The Moonstone. A Romance. […], volume III, London: Tinsley Brothers, […], published 1868, page 195

His nostrils flared ominously and his fists opened and closed at his sides.

1983, Patricia Hagan, Golden Roses

However, the next day dawned ominously with an overcast sky and we postponed our start till 8 o'clock.

1999, Harish Kapadia, “Accident and Rescue in the Nanda Devi Sanctuary”, in Across Peaks & Passes in Kumaun Himalaya, New Delhi: Indus Publishing Company, page 70