my idioms

Water Under The Bridge
We say "water under the bridge" when talk about situation in the past we have decided to forget.
"Aren't you still angry about what he said?"
"No, that was a long time ago. It's all water under the bridge."

All greek
Something is all Greek to you when it is so hard to understand that it is like reading a foreign (Greek) language.
"Can you read these instructions?"
"No. It's all Greek to me."