in. to inform (someone about something). : Who squealed to the cops?
Dictionary of American Slang and Colloquial Expressions by Richard A. Spears.Fourth Edition. Copyright 2007. Published by McGraw Hill.
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c.1300, probably of imitative origin, similar to O.N. skvala "to cry out" (see squall (v.)). The sense of "inform on another" is first recorded 1865. The noun is attested from 1747.