1. stating something quickly and without thinking hard about it; in an impromptu manner

1. Right off the top of his head he listed all the state capitals in alphabetical order.

2. "What's the capital of USA?" "I don't know off the top of my head, but I could go and look it up."

3. 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.

4. Tom: What time does the morning train come in? Bill: Off the top of my head, I don’t know.