How about this? "When pigs fly". This means "never".

"I've Gone Blank" We use this expression to say that we can't remember a piece of information or can't decide what to write or say in a situation..

Speaking Texan:. If you have ever seen a western movie, you know that cowboys wear big hats. When somebody tries to act like they're important or rich but have nothing to back it up, you can say he is all hat and no cows........ Pigs are messy and aggressive. They have absolutely no manners. When someone says they are going to be late, especially when they are invited to eat, instead of inconveniencing a whole group to satisfy the one individual, you can say: We will wait for you like one pig waits for another. In other words not at all..... Someone asks how you are, you can reply: I'm as fine as frog hair. Of course frog hair doesn't exist so it is so fine you cannot see it..... Dog fights are messy and vicious. When someone asks you to take sides in a dispute, you can say: I don't have a dog in that fight. That means that you aren't going to take sides because you have nothing at stake in that argument..... When somebody is buying a horse, an expert can look at the horse's teeth and judge the health, age and well-being of the animal. When someone offers you a gift, it is impolite to examine it to determine its value. You should just accept it with gratitude. The expression is, don't look a gift horse in the mouth.

Its just a "piece of cake", means easy one

I rarely used idioms in general talk or you can say Once in a blue moon.