Assim de cabeça, eu não sei: como dizer isto em Inglês?

You're talking to your friend about music and he's making you all these tricky questions. He wants you to tell him who's the best jazz singer from the seventies, the song that became very known from the so-and-so movie and you don't know all the answers to these questions. So you say: Off the top of my head, I don't know.

When you say something off the top of your head, you don't give it too much thought, you say it without thinking too much about it. You're only saying it based on what you remember. If you remember nothing, you say off the top of your head, you know nothing. If you have a vague memory of it, you say it the same way. We're caught off guard many times when we're asked about things we don't quite know. Just imagine yourself on the streets when someone asks you where the nearest drug store is. You really don't have a clear idea of that since you don't walk around that area very often, so off the top of your head, you give the person a vague idea of where the nearest drug store might be. Take a look at these other examples:

• 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. Tom: What time does the morning train come in? Bill: Off the top of my head, I don't know.
• 'What was the name of that plumber you used?' 'I couldn't tell you off the top of my head.' Off the top of my head I could probably only name about three women artists.