When a person is so annoying that they get you really mad and the only way to stop them doing what they are doing is to give them a single powerful punch to the face which knocks them unconscious, they are now said to be "knocked the fuck out". This is only achieved if it is a single punch which knocks them unconscious.
That bitch who tried to steal my car? I chased him down the street and punched him in the face, then his head hit the brick wall and he dropped. Now THAT'S knocked the fuck out!
Knock the fuck out, a term used mainly when someone gets punched in the face, thereby resulting in a minor (or major) concussion or simply broken/bloody nose.
Nearly any sort of head-trauma, so long as it's very painful, is considered 'getting' knock the fuck out. You never use 'knock the fuck out' with an 'ed' at the end, because it ruins the point of the gangstuh-speak. Honestly, don't add the 'ed' or you get knock the fuck out.
Me: You better get me my weed by Monday.
John: Or what?
Me: OR YOU GET KNOCK THE FUCK OUT, THAT'S WHAT NIGGA!!
(That Monday)
Me: Bitch you got my shit?
John: No, bitch.
Me: (Punches John in the face.)
Innocent bystander: Oh shit! You just got knock the fuck out!