To make great or greater; to increase in size, importance, or quality.
From the adjective great plus the verb-forming suffix -en (a productive suffix in Germanic languages). This follows the pattern of darken, widen, and deepen, though 'greaten' is archaic or rarely used.
Words with the -en ending (like 'strengthen' or 'enlighten') are survivors from Old English that created verbs from adjectives, and 'greaten' followed this perfect pattern—but English speakers preferred 'make great' instead!
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