Very pale or grayish in color, or made of ash; often describing a person's face when they're shocked, frightened, or ill.
From ash (Old English 'æsce') plus -en, a suffix forming adjectives meaning 'made of' or 'resembling,' so literally 'made of or resembling ash.'
Describing someone's face as 'ashen' when they're shocked is brilliantly visual—we see the red blood drain away, leaving a grayish pallor that actually looks like ash, which is why this metaphor works so perfectly.
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