Someone who informs authorities about wrongdoing; a person or animal that makes squealing sounds.
From 'squeal' (making a high-pitched sound, imitative origin) + '-er' (one who does). Used as slang for informant since at least the early 1900s.
In crime stories, calling someone a 'squealer' is the ultimate insult—it comes from the image of a pig squealing, suggesting the person is both noisy and a traitor!
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