US /əˈweɪ wɪð ðə ˈfɛriz/
・UK /əˈweɪ wɪð ðə ˈfeəriz/
Away with the fairies.
So as an example, "Oh my goodness, sorry, I was away with the fairies," or "Oh, don't mind Daisy, she's away with the fairies." It does come from all of those terrifying stories about children being kidnapped by fairies,