wire-tapping (n.)

also wiretapping, "surreptitiously obtaining information by connecting wires to telegraph (later telephone) lines and establishing an intermediate station between two legitimate ones," 1878, from wire (n.) + agent noun from tap (v.2). Earliest references often are to activity during the American Civil War, but the phrase does not seem to have been used at that time. Related: Wire-tap; wire-tapper.