看美剧学英语习语：Off the top of one's head

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Off the top of one's head: From memory or without much or careful consideration; In an impromptu way, without much thought; If you say something off the top of your head, you say it without thinking about it much before you speak, especially because you do not have enough time.
As in It was the best I could think of off the top of my head.

中文释义：Off the top of one's head：从字面上看，它是指在我头脑里最先想到的引申为“不假思索、马上”。Off the top of one's head 这个习惯用语产生于20世纪50年代，已经有五十多年历史了。这个习惯用美国人比英国人用得多，是说一个人对某件事没有准确的信息，只是根据自己脑子里的大概记忆或猜测来发表意见。
例句：I answered his question off the top of my head.（我没有认真考虑就回答了他的问题。）