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.