Schwamm drüber
let's forget it; let bygones be bygones
Origin
The image comes from the slate or the tavern's chalk board: with the sponge you wipe away what is written - such as a chalked-up debt - and so wipe the slate clean. The phrase was popularised among other things by a 1930s hit song.
Building blocks
Words in this expression
Examples
Das ist – !
That's long forgiven — let's just forget it!