To make dark or darken something, a rare or archaic verb meaning to obscure or dim.
From en- (cause to become) + dark (absence of light, from Old English deore). This follows the Old English pattern of adding en- to adjectives to create verbs meaning 'to make' that quality, like enable or enlarge.
Old English writers loved creating verbs by adding 'en-' to adjectives—you can still do it today to say things like 'enlighten' or 'enlarge,' but 'endark' is so obsolete that modern English speakers just say 'darken' instead.
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