A dead tung is one that no longer has any born speakers. Most often this happens when a tung undergoes speech death while the speakers swap their old tung for a new one. For show: Theedish was forsaken for Old English, and many American tongues which were forsaken for English, Spanish, French, and Portuguese.
A tung which has born speakers is known as a living tung, of which there are about 6,500 today.