the top of one's head是一个人的头顶。off the top of one's head的意思是，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，即马上要说的话，不假思索地。

【例句】
I can’t give you the figure off the top of my head.
我不可能不假思索地把那数额告诉你。

Gold is doing really well these days. It is almost $500 an ounce and experts say that it will continue to rise. I can't be sure, but off the top of your head, I'd say that it hasn't been so high since the early 1980s.
最近黄金价格很好， 已经涨到差不多五百美元一盎司了。 专家们说， 黄金价格还会上涨。 我可不太肯定， 我估计现在的黄金价格是二十世纪八十年代以来最高的。