A person who gives impromptu legal or professional advice from a street corner or casual setting, without proper authority or qualification.
From 'curbstone' (literal street corner) plus the agent suffix '-er', referring to someone offering unofficial advice from a casual public location.
A 'curbstoner' is 'curbstone lawyer' compressed—it reflects the old anxiety that anyone could dispense bad advice from a street corner, which is hilarious because now people get legal advice from Reddit and YouTube instead.
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