The phrase relates to a puzzle hint encountered when solving crosswords. The solution sought typically refers to a concealed listening device surreptitiously installed on a telephone line or within a telephone handset. For example, a crossword puzzle might have the clue “Hidden eavesdropping device on a landline,” with the answer being a three-letter word meaning such a device.
The prevalence of this type of clue reflects both the cultural significance and the historical usage of wiretapping as a method of espionage and surveillance. From Cold War-era spy dramas to contemporary thrillers, the concept of clandestine monitoring of phone conversations has permeated popular culture. Consequently, this vocabulary has become fairly common knowledge, making it suitable for crossword puzzle creators.