Archaic past tense of 'burthen,' meaning to load with a heavy burden or to oppress.
From burthen (archaic form of 'burden') + '-ed' suffix for past tense. This represents older English verb inflection patterns.
In older literature, you'll see characters 'burthened' with grief or duty—it's the past tense that Shakespeare's audiences would have used, making poetic lines like 'sore burthened with care' feel natural to their ears!
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