Green’s Dictionary of Slang

squeal (on) v.

[late 19C+ use mainly US, but note Edgar Wallace title, The Squealer (1927)]

1. [mid-19C–1920s] to complain.

2. [mid-19C–1940s] to own up.

3. [mid-19C+] to inform against one’s partners, esp. partners in crime.

4. [late 19C+] to report a crime to the police or any other authority.

In compounds

squeal rule (n.) (also squeal law)

[1960s+] (US) the law requiring parental notification when an underage girl applies for a prescription for contraceptives.