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。”
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