Canadian slang;

to knock the fuck out is offering to perform oral sex. Euphemism common among the bath-houses of Toronto and Craig's List. This is unconfirmed.

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.