"I'm going to pretend I didn't hear that." you can say this if someone tells you something you don't want to know. It implies that you are going to act like you didn't hear what the person said, even though you did.
"TMI" is an abbreviation of "too much information". It is a very common slang word especially among young people. It is said when someone talks about themselves and what they say is too personal. For example, if someone was talking about going to the bathroom you could say "TMI!".
"I'm going to pretend I didn't hear that."（聞かなかったことにします）
- これは誰かから知りたくなかったことを聞いたときに使えます。これは「それを聞かなかったかのように振る舞います」という意味です。
"TMI" は "too much information" の省略です。これは特に若い人の間でよく使われるスラングです。これは、人が自らについてのあまりに個人的な情報を伝えてきたときに使います。例えば誰かがトイレに行くことについて話していたら、"TMI!"（そこまで話さなくていい）と言えます。
The two sentences you see provided above are terrific ways to express to your listener that you wish you did not hear or know something. In the second sentence you will see the word erase. This means to remove or do away with. This word would make an excellent addition to your vocabulary because it is common in our everyday conversation.
It happens many times, unfortunately, that we see, hear or come to find out something that we prefer to not have hear, seen or known. In the US we have started using "I can't unsee that" when we see something that we wish we hadn't. If the fact is not specific to sight, keep it general and say I wish I didn't know that or I prefer to have not known that. If you "know" something, it is not specific as to how you knew it, you have either seen or heard it. You can be specific though and say I wish that I had never heard that fact or I wish that I had never seen that happen. Also you can directly say to the person freely sharing information, I really wish you had never told me that. But when they tell you, you can say right away I wish you didn't tell me that. Also I wish I didn't see that is for when the fact has just happened.
残念ながら、知りたくなかったことを知ってしまうことはよくあります。アメリカでは最近、見たくなかったものを見てしまったときに、"I can't unsee that"（記憶から消せない）というフレーズが使われます。
それが目で見たことに限定されないなら、より一般的に、
I wish I didn't know that（知りたくなかった）
I prefer to have not known that.（知りたくなかった）
と言えます。
"know" はどのように知ったか（聞いたのかあるいは見たのか）について曖昧です。
具体的に次のように言うこともできます。
I wish that I had never heard that fact（聞きたくなかった）
I wish that I had never seen that happen（見たくなかった）
また、その情報を伝えてきた人に対してすぐに "I wish you didn't tell me that"（教えてもらいたくなかった）と返すこともできます。
また、"I wish I didn't see that"（見たくなかった）、これも何かを見てしまった直後に言えます。
There is a saying when you didn't want to know something and that is 'to be kept in the dark'
'I wish I had been kept in the dark about the test tomorrow'
to be kept in the dark' は「知りたくなかった」と表す時に使える英語のイディオムです。
'I wish I had been kept in the dark about the test tomorrow'
「あしたのテストのことは知りたくなかった」
If you wanted to explain that you heard something you now wish you hadn't, you could say any of the following: "I wish I hadn't known", "In hindsight, I wish I had not been told that" or "That is information I wish I didn't know". Any of these sentences adequately explain that you wish now that you did not know this information.
聞きたくなかったこと聞いてしまったと伝えたいなら、次のように言えます。
"I wish I hadn't known"（知りたくありませんでした）
"In hindsight, I wish I had not been told that"（後から考えると知りたくありませんでしたね）
"That is information I wish I didn't know"（それは知りたくありませんでした）
上記どの文を使っても、その情報は知りたくなかったと伝えることができます。
イギリス 
日本 
アメリカ合衆国 
