What is the origin of the idiom "put a bug in someone's ear" and when to use it?

The origin of the idiom "put a bug in someone's ear" is uncertain, but it likely stems from the world of espionage and surveillance. The phrase draws upon the concept of covertly planting a listening device or "bug" to eavesdrop on conversations and gather information. It is commonly used when there is a desire to plant a seed of thought or influence without being overt or direct.