1. A stool pigeon; betrayer; an informer; one who secretly aids the police to apprehend criminals. 2. To assist authorities secretly in bringing criminals to justice. 3. To act as a stool pigeon or informer for officialdom, either openly or covertly. "If he rats on me, I'll mark his kisser (cut his face). With my bit (long prison term), I got nothing to lose."

- american underworld dictionary - 1950