A person or thing that burdens; someone who places a load or responsibility on another.
From 'burden' plus the agent suffix '-er,' which creates nouns meaning 'one who does X.' This pattern is productive in English (runner, writer, baker).
The word 'burdener' is rare today, but it reveals an elegant fact about English: we can add '-er' to almost any verb to create a person-noun, which is why our language can express so many roles and relationships!
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