Military term originally used to detail radio reception between pilots and ground crews. The ground crew outside the aircraft would initiate a radio/comms check during pre-flight checks. The pilot would then hold up 1, 2, 3, 4, or 5 fingers to the ground crewman to describe the reception. A response of 1 by 5 is poor with a response of 5 by 5 being the best. Today, it is not used in radio comms as much as it is as a slang phrase to mean "I hear you" or "I copy".
