How Well Do You Know English Slang?
c.1300, probably of imitative origin, similar to Old Norse skvala "to cry out" (see squall (v.)). The sense of "inform on another" is first recorded 1865. The noun is attested from 1747.
An informer; rat, snitch, squeal (1865+)
To inform; rat, sing, squawk (1825+)