off the top of your head idiom
凭既有知识；不假思索地
from the knowledge you have in your memory
"What's the capital of Mauritania?" "I don't know off the top of my head, but I could go and look it up." “毛里塔尼亚的首都是哪里？”“我没法马上告诉你，但我可以去查一查。”
江苏热线英汉汉英双解词典包含207361条英汉翻译词条，涵盖了常用英语单词及词组短语的翻译及用法，是英语学习的必备工具。