Past tense of 'rat': to betray someone's secrets or wrongdoing, usually by telling an authority figure; or to hunt rats.
From Old English 'ræt' and Middle English 'rat'. The sense of 'betrayal' emerged in underworld slang around the 1800s, likely because rats were seen as sneaky and disease-carrying vermin.
Calling an informant a 'rat' is one of the oldest insults because medieval cities were plagued by actual rats carrying disease and destroying food stores—so 'ratting someone out' literally equated a person to a destructive pest.
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