to space something out 彻底忘记（俚语）

英文释义

To forget something completely due to distraction, fatigue, or mental confusion.

例句

I have been so busy at work that I spaced out my mom's birthday, which is tomorrow, and I need to buy her a present.

我工作实在太忙，竟然忘了妈妈明天过生日，我必须给她买个礼物。