Green’s Dictionary of Slang

(US) to act as an informer.

1958		C. Himes Real Cool Killers (1969) 47: ‘If I thought you’d pigeon, I’d kill you.’ ‘You know I ain’t gonna pigeon, Sheik. You know me better than that.’.
1965		C. Himes Cotton Comes to Harlem (1967) 71: Everyone knew she was a stool pigeon; but she pigeoned on the police too.