A person or thing that furthers something; one who helps advance a cause or goal.
From the verb 'further' plus the agent suffix '-er,' creating a noun meaning 'one who furthers.' This follows standard English word-formation patterns.
Though rarely used today, 'furtherer' was a common word in older English to describe someone dedicated to a cause—think of abolitionists called 'furtherers of freedom,' but we'd just say 'advocates' now.
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