To make or become larger; an archaic or dialectal form meaning to increase in size or magnitude.
From 'big' with the causative suffix '-en', similar to how 'darken' comes from 'dark'. This formation is archaic in modern English.
Biggen shows an old English pattern where you could add '-en' to adjectives to make them verbs—think 'frighten', 'lighten', 'quicken'—a word-building rule we've mostly abandoned!
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