informal, spoken
from the knowledge you have in your memory,
based on what you remember,
without giving it too much thought or without precise knowledge,
if you say something off the top of your head, you say it without thinking about it for very long or looking at something that has been written about it
Off the top of head - Idioms by The Free DictionaryExamples:
[1]
Mary: How much do you think this car would be worth on a trade?
Fred: Well, right off the top of my head, I'd say about a thousand.
[2]
Tom: What time does the morning train come in?
Bill: Off the top of my head, I don't know.
[3]
I couldn't tell you what the final score was off the top of my head. Off the top of my head I could probably only name about three women artists.
[4] "What's the capital of Mauritania?" "I don't know off the top of my head, but I could go and look it up."