1) BATS IN THE BELFRY
The old lady has bats in her belfry. Everyone thought I had bats in my belfry but I promise you I was telling the truth. Anyone who thinks he can fly probably has bats in his belfry.
MEANING: Someone with bats in the belfry is crazy or eccentric. To be crazy or eccentric.
ETYMOLOGY: Dates back to1899. Come from the tendency of bats to fly around erratically, with "belfry" indicating "head, mind". belfry noun - a bell tower or steeple housing bells, esp. one that is part of a church. - a space for hanging bells in a church tower.
2) BIGGER FISH TO FRY
I asked Steve to help me, but he said he had bigger fish to fry.
I'd love to stay and chat, but i've got bigger fish to fry.
Dave wants to apply for that job, but he has bigger fish to fry at the moment. MEANING: If you aren't interested in something because it isn't important to you and there are more important things for you to do, you have bigger fish to fry.
ETYMOLOGY: Frying fish is here used as a metaphor for the things one does. Thus to say that one has bigger fish to fry means that someone has more important things to do. the bigger the fish, the more important it is.
3) BY A WHISKER
David won the game only by a whisker. I'm lucky to be alive. The bus missed me by a whisker. Last time she raced against the Brazilian she won by a whisker. He missed the goal by a whisker.
MEANING: If you do something by a whisker, you only just manage to do it and come very near indeed to failing. So it means 'by a very small amount'.
ETYMOLOGY: A whisker, like any hair, is very fine and very small. Therefore it's used to convey the idea of things happening by a small amount.