write it off

不要放在心上了。

If you write someone or something off, you decide that they are unimportant or useless and that they are not worth further serious attention.

认为....不重要或者无用，不值得再去注意或者在意

write意为“写”，off是“抹掉”，把写的东西抹掉，引申为“不要放在心上”。当你的朋友因为不开心的事而烦恼时，可以用这句话来安慰他。

Lily: I made a mistake over the bill yesterday. I should take the blame.

莉莉：我昨天把账算错了，我要为过失承担责任。

Jane: Write it off.

简：不要放在心上了。