off the top of one's head 信口开河

【讲解】

此语意为 if you answer a question or provide information off the top of your head, you do it immediately without checking the facts （大致猜想,随兴而发）。

【对话】

A:Bob, can you tell me how many students passed the college entrance examination this year?
A:鲍伯，你能告诉我今年有多少人考上大学了吗？
B:Off the top of my head, I'm afraid I don't know.
B:不查的话，我也搞不明白。
A:Never mind. Well, can you tell me where I can get the information?
A:不要紧，你知道从哪里能查到有关的信息吗？
B:Maybe from the Education Daily.
B:《教育报》上也许会有。