Definition of listen in

The phrase "listen in" originated in the early 20th century when the technology for intercepting and monitoring wireless telegraphy and telephone conversations became available. The term referred to the act of secretly listening to communications between two or more parties without their consent. The origin of the phrase can be traced back to the early days of wireless telegraphy, when the executives of competing companies would surreptitiously listen in on their rival's transmissions to gain a competitive edge. As the technology for intercepting wireless transmissions improved, so did the phrase's usage, and it soon became applied to other forms of communication, such as telephone conversations. The use of the term "listen in" became increasingly popular in the 1960s and 1970s, particularly in reference to law enforcement and intelligence agencies monitoring conversations in their efforts to uncover criminal activity or prevent national security threats. Today, the phrase is commonly used in everyday conversations to describe the act of eavesdropping on a conversation without being invited to do so. In summary, "listen in" originated in the early 20th century as a term to describe the act of covertly listening to wireless transmissions, and its usage was expanded to include telephone conversations as technology advanced. Today, the term remains a part of popular vernacular, even as advances in technology have made it easier for individuals to conduct such activities.