a stool pigeon
An informant, especially a criminal working as a spy for law enforcement officers. Although the criminal agreed to help the police in order to avoid prison time, he was afraid the other gang members would kill him if they discovered he was a stool pigeon.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
a stool pigeonOLD-FASHIONED
A stool pigeon is someone who gives secret information to the police. There's nothing the mob hates more than a stool pigeon. Note: This expression is used to express disapproval. Note: This expression comes from the old practice of putting a wooden pigeon on a seat to trick other pigeons.
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012