off the top of your head from the knowledge you have in your memory 凭既有知识;不假思索地 A：What's the capital of Mauritania?" 毛里塔尼亚的首都是哪里? B：I don't know off the top of my head, but I could go and look it up. 我没法马上告诉你，但我可以去查一查。