Sibylline is characteristic of, or relating to a sibyl or prophetess. Sibylline language would be oracular and/or represent that which has a secret or hidden meaning. The oracular seeresses known as the Sibyls of antiquity prophesied at certain holy sites under the divine influence of a deity. Plato only speaks of one Sibyl, but in course of time the number increased to nine. To the classical sibyls of the Greeks, the Romans added a tenth. These ten were the Babylonian or Persian Sibyl, the Libyan, the Cimmerian, the Sibyl of Delphi, the Erythraean, the Samian, the Cumaean, the Hellespontine, the Phrygian and the Tiburtine. Long after the oracles had long been silenced by the Christians in the 4th century, the number of Sibyls was increased in the Middle Ages to as many as twelve.
The Sibylline Books of Roman history were a collection of oracular utterances, set out in Greek hexameters and were lost twice, thus there is very little knowledge of the actual contents. The Sibylline Books should not be confused with the Sibylline Oracles which have survived. The Sibylline Oracles, written in Greek hexameters contain a medley of pretended prophecies by various authors and of various dates, from the middle of the second century B.C. at the earliest, to the fifth century A.D.