A supporter, promoter, or one who favors something; an advocate or patron, especially in legal or official contexts.
From Latin fautor, from favere meaning 'to favor' or 'to support.' The word appears in legal documents and formal writing as a person who promotes or backs a cause, person, or action.
In old legal documents, if someone was called a 'fautor' of a rebel or criminal, you basically had a target on your back—you were guilty by association for supporting the wrong person!
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