The word Sibylline has origins in the Greek word Sibulia, meaning "prophetess."  That was the picked up by the Latin word Sibillinus, from Sibylla (sibyl).  Back in the times of the Greeks and Romans, a sibyl was a female oracle who delivered cryptic prophecies foretelling the future, often inspired by the deities. The adjective sibylline describes such talents or the messages they reveal.  That was then picked up by the French (circa 1200) sibile, sibille, then to English, Sibylline in the late 16th century. 

The earliest recorded use of the word "sibylline" in English is from around 1570-1580, according to both Dictionary.com and the Oxford English Dictionary.  OED's earliest evidence for Sibylline is from around 1579–80, in a translation by Thomas North, translator. Sibylline is a borrowing from Latin. 

Sibylline first used in English: 
"It took several historians to understand the sibylline meaning of the ancient scroll." 
"Although the sibylline book was meant to have a secret meaning, the code was cracked long ago." 
"Sibylline messages were passed between the secret spies." 
"Cryptic and sibylline, the scientists have not figured out the what the mysterious drawings on the cave wall mean." 
"Many believe that the sibylline crop circles in the middle of fields are secret messages from aliens." 
"It's confessional and ruminative, yet also clipped, ‘teacherly’ and sibylline." 

Just the facts:

SIBYL Definition & Meaning - Merriam-Webster

Sibylline - Definition, meaning and examples | WordUp App