adverb, adverb taking the 'to' particle, noun or participle which takes 'suru'
sullenly; silently in a bad mood; gloomily
Describes someone who is visibly displeased or in a bad mood but remains silent, often with a sulky expression. Common with する or と.
彼は朝からむっすりしている。
He's been sullen since morning.
He fell into a gloomy silence.