"lour : (intransitive) To frown; to look sullen. (intransitive, figuratively) To be dark, gloomy, and..."

“lour : (intransitive) To frown; to look sullen.
(intransitive, figuratively) To be dark, gloomy, and threatening, as clouds; of the sky: to be covered with dark and threatening clouds; to show threatening signs of approach, as a tempest.”

- https://en.wiktionary.org/wiki/lour#English

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