Past tense of squeal: made a high-pitched cry or sound, or informed on someone to authorities.
From Middle English 'squelen', imitative of a high-pitched sound. The 'informer' sense developed from the similarity between the sound of distress and the act of betraying someone under pressure.
The connection between making a high-pitched sound and betraying someone shows how onomatopoeia can develop metaphorical meanings. The phrase 'squeal like a pig' combines both the literal animal sound and the figurative sense of revealing information.
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