means To make a high, shrill cry — or, informally, to inform on someone to those in authority.

from An echoic word, born to imitate the very sound it names: that thin, piercing cry. It surfaces in Middle English as a cousin to 'squeak' and 'squall,' part of a noisy family of words that copy sharp sounds. The 'informer' sense — to squeal on someone — is a later slang twist, the idea being that a betrayer cries out and gives the game away, like a pig under pressure.