off the top of one’s head 美国俚语，表示不假思索地；即兴地；随口而出地

【例句】
When the teacher called my name, I was frozen. Then standing up to recite the poem, I found myself doing it off the top of my head! Can you believe that?
当老师叫我名字的时候，我真是愣住了。然后，我站起来背那首诗，我发现自己不加思索地。你相信不相信？

Sorry, I can't give you the exact figure off the top of my head.
对不起，我现在一下子不能给你精确的数字。