In a manner that causes gloom or displays gloomy characteristics; darkly or melancholically.
From the gerund 'glooming' plus '-ly,' an unusual construction creating an adverb from a gerundial form, making it a somewhat literary or archaic formation.
This is a rare construction—stacking '-ing' and '-ly'—which shows how English speakers can layer suffixes creatively for subtle emotional effects, especially in poetry and literature.
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