LISTEN IN (ON) is a phrase that refers to the act of secretly listening to a conversation without the knowledge of the people involved. WIRETAP, with 7 letters, is a method of secretly listening to a conversation by tapping into a telephone line. EAVESDROP, with 9 letters, is a method of secretly listening to a conversation by standing close to the people involved. SNOOP, with 5 letters, is a method of secretly listening to a conversation by searching through someone's personal belongings.