A slap on the head, usually the back of the head. Not hrd enough to really hurt, but suffient to shake up the recipient more than a little.
Often used in a group of males to "encourage" the one that's always a bit slower than the others.
"So that's three Big Macs, a Quarter pounder, a chicken sandwich .... Jim, what do you want ?"
"Err... a Big - no, one of those fish .... can I have nuggets ? No hang on, something with bacon ....."
"Make your mind up NOW, unless you want some more brain therapy !"
"Ok, OK, Big Mac and Fries !"