1
general::
Phrase(s): (right) off the top of one’s head
Fig. without giving it too much thought or without precise knowledge. • 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.
McGrawhill's American Idioms And Phrasal Verbs