For my Troll Wife story, I dug through old and lesser known Grimm's tales for some hints of expressions that faeish might use. In my world building, I've already established that many fairy tales are faeish in origin, and usually have references to the war between humans and faeish. This made it a natural to go back to those old tales for Oubliette's expressions. I have some requirements:
1. It must be understandable, even if you don't know where the expression comes from. I don't want to have to explain them to someone, so that the reader will know what they mean. That kind of exposition bugs the heck out of me.
2. I may create a lot of them, but I can't use too many in the book, or it will come off badly.
3. Oubliette is a very angry monster. She probably won't be using funny and lighthearted expressions, only the depressing ones.
So, here's the list I've gotten so far (will be expanded):
1. "When the dry twig sprouts green", as in "You can have that when the dry twig sprouts green" It means "never" or "highly unlikely".
2. "The mercy of a mill stone", as in "Oubliette has the mercy of a mill stone". It means "none". Mill stones were a human invention, and are seen as a metaphor for the way humans grind everything beneath them.
3. "The rabbit that cries for the moon", as in "No, you can't have that. Don't be the rabbit that cries for the moon." "Rabbit" is a term that faeish use for their children, and the expression means someone wanting something they can't have, either a child, or someone that is acting like a child.
4. "A hedgehog bride", as in "That couple is well matched, she is a hedgehog bride". It means not to love/marry someone that is much prettier or uglier, dumber/smarter, etc., than you are. "Box your weight" is a human expression for the same kind of thing. (This one may be too obscure. I may not use it.)
5. "His name is carved in stone", as in "I will not rest until his name is carved in stone". It means dead, of course, and refers to the custom of carving names in a gravestone. It sounds like something I might read in a Western, so I think this one needs work.
Now I need to think of more situations that call for these. Like I said, even if I don't use them in the book, they're awfully fun to create :)