Becoming dark or tending toward darkness; growing darker in color or light.
From 'dark' plus the present participle suffix -ing. This is a productive formation in English, where -ing creates adjectives from verbs, used since at least medieval times.
While 'darkening' is much more common, 'darking' is an archaic poetic way to describe the moment when light fades—you might find it in older English literature painting a mood of approaching dusk or sadness.
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