expression, godan verb (-su)
Describes a facial expression of worry, displeasure, or deep thought, literally 'to cloud one's eyebrows'.
彼は難しい質問に眉を曇らせた。
He knitted his brows at the difficult question.
心配そうに眉を曇らす母親の顔が忘れられない。
I can't forget my mother's face, frowning with worry.