prophetic; oracular

According to the Merriam-Webster dictionary, a sibyl is “any of several prophetesses usually accepted as 10 in number and credited to widely separate parts of the ancient world (as Babylonia, Egypt, Greece, and Italy).” Here is the quote from The Double: “[He] was convinced that such a metaphorical, sibylline conclusion would remove any possibility of the company forwarding the letter to an actor…”