A person who gives or stands as a pledge or security; someone who provides a bond or guarantee for another person's actions.
From caution + -er suffix denoting an agent. Used in Scottish and legal English to describe someone providing cautio (a formal pledge).
In Scottish law, being a 'cautioner' was serious business—if the person you vouched for committed a crime or broke an agreement, YOU paid the penalty. It's why people chose their cautioners very carefully.
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