A joint snake is a mythical creature of the American southern states, the myth likely having spread elsewhere.
Supposedly, the snake can break itself (or be cut) into pieces and will reassemble itself. It is said that if a piece of the snake is taken and the pocket knife used to cut the snake is set down in in the place of the snake's piece, the knife will join up with the whole of the snake.
The myth is probably based on legless lizards than can regenerate their tails after they are broken off. Such lizards are often called joint snakes. May also be a reference to the Hydra in Greek Mythology.