It slipped my mind.

我忘记了.

slip your mind:if something slips your mind, you forget it or forget to do it 被遗忘
I'm sorry I didn't tell you. It completely slipped my mind.
抱歉,我没有告诉你.我全给忘了.



spill the beans:to tell somebody something that should be kept secret or private 泄露秘密;说漏嘴

Billy spilled the beans, so we all had to stay after school.
比利泄露秘密,所以放学后我们都必须留校.
He is sure to spill the beans before long.
用不了多久他准会走漏消息的.



get/have cold feet:to suddenly become nervous about doing something that you had planned to do 临阵胆怯;畏缩

He was going to ask her but he got cold feet and said nothing.
他本来是想向她提出邀请的,可事到临头他却胆怯得什么也没有说.



wet blanket:a person who is not enthusiastic about anything and who stops other people from enjoying themselves 泼冷水的人;扫兴的人

Don't bring Robert to the picnic. He's such a wet blanket.
不要带罗伯特去参加野餐,他实在是个很扫兴的人.



come clean:全盘托出,招供

He came clean and confessed the whole thing.
他招供并承认一切罪行.
John came clean about his own views.
约翰把自己的看法全盘托出.



by the book:following rules and instructions in a very strict way 循规蹈矩;严格遵守章法

She always does everything by the book.
她总是照章行事.



gimmick:(为引人注意或诱人购买而搞的)花招,把戏,噱头

a promotional/publicity/sales gimmick
以推销／宣传／销售为目的的花招
We don't use gimmicks to sell our products.
我们销售产品不搞什么噱头.



get in the way of:to prevent somebody from doing something; to prevent something from happening 挡…的路;妨碍

He wouldn't allow emotions to get in the way of him doing his job.
他不会让感情妨碍自己的工作.



go out of your way (to do something):to make a special effort to do something 特地;格外努力

He would always go out of his way to be friendly towards her.
他总是特意向她表示友好.



have it your own way!
used to say in an angry way that although you are not happy about something that somebody has said, you are not going to argue 随你的便好了

Oh OK, then. Have it your own way.
啊,好啦,随你的便吧.