From Mackey’s Encyclopedia of Freemasonry:
A term of incantation which was formerly worn about the neck as an amulet against several diseases, especially the tertian ague (known today as tertian malaria).
It was to be written on a triangular piece of parchment in the following form:
It is said that it first occurs in the Carmen de Morbis et Remediis (The full text being called De Medicina Praecepta Saluberrima- Carmen de Morbis et Remediis) of Q . Serenus Sammonicus, a favorite of the Emperor Severus in the 2nd and 3rd centuries, and is generally supposed to be derived from the word abraxas (ΑΒΡΑΣΑΞ).
You can find the text of De Medicina Praecepta Saluberrima- Carmen de Morbis et Remediis here.