means an analog device that mimicked telephone switching tones to hijack the phone network and place free long-distance calls.

from emerged in the 1960s after phreakers realized the phone network's call-routing signals traveled as audible tones, right in the same line you were talking on. a whistle from a cereal box toy could hit 2600hz and trick at&t's switches into thinking the line was idle, opening the door to free routing.