Away with the fairies
Fairies are magical creatures and children in the UK love the tooth fairy who exchanges any teeth they leave under their pillow for money - a £1 coin or maybe more if they’re lucky! If you’re away with the fairies it means that you are daydreaming and your mind is elsewhere. It’s like it’s gone on a journey to another planet. Finally, you can also eat fairy cakes which are small sponge cakes baked in paper cases - delicious!
Example
“My neighbour keeps leaving her keys in the front door. She’s away with the fairies most of the time.”