In the most basic of explanations, it means to cause a sex-induced coma. The goal is to fuck someone so well, that they fall to sleep and are late for work on a Monday when it's only Saturday.
MAN CALLING HIS GIRLFRIEND: "I hope you got another sick day, cause I'm about to knock it out."

GIRLFRIEND: "I'm on my way!" (doing 100mph)
by Moody G July 26, 2008
Get the Knock it out mug.
- Good luck with your game!
- Thanks! I`ll knock it out of the park!
by Fjall October 15, 2015
Get the knock it out of the park mug.
1. To masturbate to orgasm (usually suffixed by "over" something or somebody). This is usually but not exclusively used to describe male masturbation.
2. To lose consciousness (3rd person). Used when describing a general nonspecific occurrence.
3. To lose consciousness (1st person Royal). Used exclusively by members of the hereditary ruling families of English speaking countries to describe what happened to them when they were last playing "Who's the Tampon, Camilla?"
4. (imperative) An injunction given, in the 2nd person singular, to knock out one of whatever is to be knocked out.
1. "Did you see that .jpeg of Madonna with the thong and pickled egg? I was on my way to work but when I saw that in my inbox I was waylaid by the need to rest awhile and knock one out."
2. "Billy, have you been knocking one out over here? The smell in this room is enough to knock one out."
3. "One has been knocking one out, one must pronounce oneself feeble as the strain of so doing has been enough to knock one out."
4. When two Scouse teenagers - wearing Kappa tracksuits and sporting formidable Scouse 'fro hairstyles - are going around smashing windows of buildings, and come across a 14th century church with several irreplaceable Gothic stained glass windows depicting scenes from the life of the Virgin Mary, passersby may often hear one Scouse youth inquire of the other whether the ecclesiastical image is worthy to be spared from their brand of urban renewal. The response is frequently "A-right, we'll just knock one out, then, an' leave the rest, okay Barry?" (HMB's Guide to Liverpool Nightlife, Fodders and Stoughton, 1993)
by HMB April 6, 2003
Get the knock one out mug.
Same as "go for it" or "go nuts".

If you really dont care at all about a person doing/taking something you say "knock yourself out" as in to do/eat until you pass out (if you really want to).

Same as "go for it" or "go nuts".
Person 1: "Hey man, can I have some beer?"
Person 2: "Yeah, Knock yourself out"
by Berd May 29, 2005
Get the knock yourself out mug.
A term for: Do whatever the hell you want, it doesn't bother me.
"dad, can i go to the park?"
"Can you get there yourself?"
"Yeah"
"well then, knock yourself out."
by donnald duck June 4, 2005
Get the Knock yourself out mug.