To make greater or increase in size, strength, or importance; an archaic or rare intensification of 'greaten.'
From en- (prefix intensifying an action) + greaten (archaic verb form of 'great,' meaning to make great). This follows the Middle English pattern of using 'en-' to intensify verbs.
This word is so old and unused that it barely appears in modern English, but it shows how the prefix 'en-' was once used productively—speakers could add it to almost any adjective to create a verb meaning 'to make [adjective].'
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