off the top of one's head 信口開河

【講解】

此語意為 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,隨興而發）。

【對話】

A:Bob, can you tell me how many students passed the college entrance examination this year?
A:鮑伯，你能告訴我今年有多少人考上大學了嗎？
B:Off the top of my head, I'm afraid I don't know.
B:不查的話，我也搞不明白。
A:Never mind. Well, can you tell me where I can get the information?
A:不要緊，你知道從哪里能查到有關的信息嗎？
B:Maybe from the Education Daily.
B:《教育報》上也許會有。