It's actually multiple muscles, and the myth may have emerged because you've probably never felt your tongue get tired. Even still, it isn't the strongest muscle system. The honor of strongest single muscle likely belongs to the masseter, in your jaw. From Scientific American:
By sticking a pliable air-filled bulb into a subject’s mouth, scientists can measure the maximal pressure the tongue can exert on an object. This device, called an Iowa oral performance instrument, is placed on the tongue and subjects are asked to push it toward the roofs of their mouths as hard as they can. Scientists also use this bulb to measure endurance, or how long the tongue can hold a certain posture.