In Prince Caspian, there's a passage where Aslan apparently uses his power to summon up the spirit of Old Narnia, leading to the eventual defeat of the Telmarines with very little bloodshed. A wild party ensues, featuring Bacchus, Silenus, a lot of girls, and a lot of vines and grapes. This nod to ancient paganism within the Christian allegory of the Narnia series could be the subject of a whole other question, but here I'm asking specifically about their call, seen as follows (emphasis mine):
She never saw where certain other people came from who were soon capering about among the trees. One was a youth, dressed only in a fawn-skin, with vine-leaves wreathed in his curly hair. His face would have been almost too pretty for a boy’s, if it had not looked so extremely wild. You felt, as Edmund said when he saw him a few days later, “There’s a chap who might do anything—absolutely anything.” He seemed to have a great many names—Bromios, Bassareus, and the Ram were three of them. There were a lot of girls with him, as wild as he. There was even, unexpectedly, someone on a donkey. And everybody was laughing: and everybody was shouting out, “Euan, euan, eu-oi-oi-oi.”
I've tried to Google this phrase but found only references to this very passage in the Narnia books. Does it have any significance in terms of the mythology of Bacchus and Silenus? Lewis seems to have taken care over the writing of this passage, including a lot of hints for the educated reader on who these people are (Bromios and Bassareus are old names for Bacchus/Dionysus) before Susan and Lucy come right out and say it. So I wonder if their cry, repeated again a few paragraphs later, is also a reference to something. To me it looks both Greek, with all those vowels, and reminiscent of the traditional British "Oggy, oggy, oggy, oi-oi-oi".