2--There is nothing on her head.
 
1--But let's see if we can find out what she means.
 
2--Do you know Tom's email address?
 
1--I couldn't tell you off the top of my head, but I have it in my contacts list on my computer. Can I send it to you later?
 
2--Sure. That would be great. Thanks.
 
1--"Off the top of my head" is a common phrase that means I am trying to remember something, and to give you a quick answer.
 
2--But if a person's memory is bad, sometimes the information off the top of someone's head is wrong.
 
1--So it's best to confirm it.
 
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注解：

1.contacts联系  
2.information信息 
3.confirm确认 

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译文：

1--当你听见“Off the top of my head（未经深思熟虑不能确定）”，你知道是什么意思么？
2--她头上可没有什么东西。
1--听听下面的对话，看看能否知道她在说什么。
2--你知道Tom的邮箱地址么？
1--但是现在我未经深思熟虑，不能确定（off the top of my head）。我电脑里有份通讯录。我能晚点发给你么？
2--当然。这样太好了。谢谢。
1--“Off the top of my head”是一个很常见的短语，意思是我努力回忆某件事，未经深思熟虑很快给你答复。
2--但是如果某人的记性不好，有时候未经深思熟虑的信息是错的。
1--所以做好是确认一下。