off the top of ones head -用法

off the top of one's head

without thinking hard； quickly不加思索地

He didn't prepare his lecture beforhand， but only talked off the top of his head．他事先没有准备，在讲演时只是想到哪里讲到哪里。

Jim answered the teacher's question off the top of his head．吉姆回答老师的问题时不大动脑筋。