It simply means so many things went wrong that the person gave up. In this case he stopped trying. It comes from the business world: The Insurance company said my car has been written off [of their records] as it cannot be repaired. It means they paid the money and closed their records. We use it as a metaphor. So when so many bad things happen in a day we can write it off as a day of no sales or maybe bad feelings. It just means our plans didn't work.

As Joseph said, it comes from the business world. When a bank "writes off a loan", it means the bank accepts that the loan will never be paid and its value is zero. The bank gives up on ever being paid back. Hence, "writing off" means giving up.

Hello, It means nothing meaningful happened that day. You might as well carry on as if that day never existed. I hope this answers your question. If you’d like to practice speaking. I’d be happy to help you develop a study plan. Feel free to view my teaching profile and class offerings and send me a message if you think I can help you. Regards

Hi Zhe Zou, yes it means you didn't do anything for the whole day. Some examples: I wrote the whole day off as I couldn't be bothered doing anything. This whole day was a write off but it was worth it.