One who forswears; a person who renounces or takes an oath against something.
From 'forswear' + '-er' (agent noun suffix). This follows standard English word formation, creating a noun for the person performing the action of forswearing.
A 'forswearer' is someone who has made a binding oath to reject something—it's a moral identity, not just an action, which is why it sounds so serious and somewhat archaic in our casual modern speech.
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