A legendary Libyan serpent which has a head at each end. According to Greek mythology, the creature emerged from the blood that dripped from Medusa's head onto the Libyan desert when Perseus flew over it.
Pliny1 described it as follows:
The amphisbaena has a twin head, that is one at the tail end as well, as though it were not enough for poison to be poured out of one mouth.
Late Middle English, via Latin from Greek amphisbaina, from amphis, "both ways" + bainein, "go."