off the top of one's head

Example: “Off the top of my head, I'd say there are about forty people coming.”

Origin: American, and generally dated to the mid-20th century — an early example turns up in Harold L. Ickes's Secret Diary in 1939. The phrase describes an answer or idea given without preparation, pulled out on the spot rather than thought through in advance. Beyond the general American mid-century timeframe, no source manages to pin down exactly why the top of the head specifically became the image for spontaneous, unrehearsed thought rather than any other part of it.