Archaic present participle of 'burthen,' meaning the act of loading with or being loaded with a burden.
From burthen (archaic form of 'burden') + '-ing' suffix for present participle. This reflects older English verb conjugation patterns.
Medieval and Renaissance writers loved the flow of words ending in '-ing,' and 'burthening' sounds heavier and more dramatic than modern 'burdening,' which is why poets chose these older forms!
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