It is believed to be derived in the middle of the 20th Century when Americans started usingthis expression. Thoughts come from your brain which is in the top of your head. So if you blurt out facts quickly without having to think about them, then you are talking 'off the top of your head.'
The originate from Newfoundland and Labrador, Canada BTW I can't believe no one knew this I'm 12 and I knew this off the top of my head
Off the top of their headS because they are plural. Multiple people have more than one head...
Off the top of my head, I believe that I can.
Off the top of my head: warden
On the top, above his head.
Off the top of my head: kid who fell
9.9 off the top of my head
On the top of the head. (Anywhere else it would fall off).
Off the top of my head, 'absorb' comes to mind...
Chop it all off!!!
off the top of my head gloom and sunkern
Some Off The Top Of My Head: Icing High Sticking Tripping Blind Checking Some Off The Top Of My Head: Icing High StickingTripping Blind Checking