A head-slap occurs when someone does something that they shouldn't or that is so silly, they "need" to be hit for it. A person uses their hand to strike someone with medium strength either on the forehead or on the back of their head. V-8 is a drink that is promoted as being good for you because it's composed of vegetables that have been juiced. I believe they have fruit drinks now, too. There was an old V-8 commercial that involved someone having the option of having a V-8 or having something else. When they reached for something else, they received; a slap to the head as punishment and were told to drink V-8 because it has all of these fruits and vegetables in it that were good for you. Thus came the term "V-8 headslap." Here are some commercials: http://www.youtube.com/watch?v=J-pyGdJCV04&safety_mode=true&persist_safety_mode=1 http://www.youtube.com/watch?v=UJAJ8xw4374&NR=1&safety_mode=true&persist_safety_mode=1 http://www.youtube.com/watch?v=_VZxE6pW_to&NR=1&safety_mode=true&persist_safety_mode=1 http://www.youtube.com/watch?v=qplZbReGtr4&NR=1&safety_mode=true&persist_safety_mode=1

It's knocking of cylinder heads in a V8 engine.

A V 8 headslap =A slap upside the head Example: If he doesn't stop running around with that gang, somebody needs to give him a V 8 slap before he ends up in jail.