Definition & Meaning of "stool pigeon"

What is the origin of the idiom "stool pigeon" and when to use it?

The idiom "stool pigeon" originated in the 19th century and is used to refer to a person who acts as an informant or betrayer, particularly in criminal contexts. The term finds its roots in the practice of trapping pigeons by placing a stool or decoy bird to attract them. This idiom carries a negative connotation, suggesting duplicity and disloyalty in one's actions.