Ian: Can I borrow your car, man?

Phillip: No, dude. Last time I let you drive it, you crashed into a tree.

Ian: Come on, man. That was five years ago. It’s water under the bridge.

Phillip: No.

“Water under the bridge” is a problem between people that happened in the past. Now the people involved can forget about it and make peace. The water that flowed under the bridge in the past has now flowed away in the river and is not worth worrying about.