To become dark or to grow darker, especially gradually or subtly.
From 'dark' with the suffix '-le' (a diminutive or frequentative ending used in verbs like 'sparkle' and 'crackle'). It emerged in English poetry and literature to express the gradual process of darkening rather than a sudden change.
This is a poetic verb that Edgar Allan Poe loved—it captures something that 'darken' misses: the quality of becoming dark in a gentle, almost mysterious way rather than a harsh transformation.
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