An archaic word meaning to make completely black or to darken something entirely.
From Old English 'for-' (completely) + 'blæc' (black/pale). The prefix intensifies the coloring action, creating a verb meaning to blacken thoroughly or totally obscure with darkness.
This word shows how Old English verbs could be created almost freely—you could take any adjective like 'black' and add 'for-' to create a new verb meaning 'to make completely that color,' which is why Old English feels so creative and strange to us.
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