E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
One of the two gates of Dreams; the other is of ivory. Visions which issue from the former come true. This whim depends upon two Greek puns; the Greek for horn is keras, and the verb krano or karanoo means to bring to an issue, to fulfil; so again elephas is ivory, and the verb elephairo means to cheat, to deceive. The verb kraino, however, is derived from kra, the head, and means to bring to a head; and the verb elephairo is akin to elchus, small.
Anchises dismisses Æneas through the ivory gate, on quitting the infernal regions, to indicate the unreality of his vision.