The process of becoming darker in color, brightness, or mood.
From 'dark' (Old English 'deorc') plus '-en' (to cause to be) plus '-ing' (present participle). The word has been used since medieval times to describe both literal loss of light and metaphorical loss of hope or cheerfulness.
Notice how we use 'darkening' for mood as much as for light—'her expression darkened' and 'the sky darkened' use the same word because humans instinctively connect darkness with sadness or danger.
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