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1.
1. to be extremely knowledgeable or experienced.
2. a person who acts as though he/she knows everything and overwrites (DISMISSES) the ideas, opinions, comments and/or suggestions of others.
by Mr. Terrence L. Trezvant October 05, 2003
 
2.
pejorative One who makes it appear as if they're an expert on a subject while there are also signals (subtly or blatantly noticeable) that this person doesn't know very much on the subject or at least as much as they're acting like they do on the subject; pretending to have much knowledge on a subject matter
(Note: there are countless ways to seem knowledgeable on a subject as by behaving correctively and correcting people all the time, giving unwanted advice, expressing opinions as fact, not accepting advice or information from others, etc., but when there are also indicators that one doesn't know what they're talking about, they're considered a know-it-all or engaging in know-it-all behavior)

"Even though I'm only fifteen and I've never held a job before I want you to listen to me carefully; on your interview, if you're not sitting cross legged, they won't hire you. If you don't nod, they won't hire you. Employers hate when you don't wear a tie. Companies these days aren't into what's on your application. Employers these days are real into the whole look aspect of it all. Take it from me," said my know-it-all son before I handed him his schoolbag and replied, "Shut up and go to school, dear!"

I found it shocking that some know-it-all bum out on the street was telling me how to handle my taxes the right way.

I try very hard not to seem like a know-it-all when discussing scientific matters; I provide evidence and back up what I'm saying and explain to others of my college experience in the field. That way, people regard me as knowledgeable and not a know-it-all.
by Rick Warwick August 27, 2008
 
3.
pejorative One who makes it appear as if they're an expert on a subject while there are also signals (subtly or blatantly noticeable) that this person doesn't know very much on the subject or at least as much as they're acting like they do on the subject; pretending to have much knowledge on a subject matter
(Note: there are countless ways to seem knowledgeable on a subject as by behaving correctively and correcting people all the time, giving unwanted advice, expressing opinions as fact, not accepting advice or information from others, etc., but when there are also indicators that one doesn't know what they're talking about, they're considered a know it all or engaging in know it all behavior)

"Even though I'm only fifteen and I've never held a job before I want you to listen to me carefully; on your interview, if you're not sitting cross legged, they won't hire you. If you don't nod, they won't hire you. Employers hate when you don't wear a tie. Companies these days aren't into what's on your application. Employers these days are real into the whole look aspect of it all. Take it from me," said my know it all son before I handed him his schoolbag and replied, "Shut up and go to school, dear!"

I found it shocking that some know it all bum out on the street was telling me how to handle my taxes the right way.

I try very hard not to seem like a know it all when discussing scientific matters; I provide evidence and back up what I'm saying and explain to others of my college experience in the field. That way, people regard me as knowledgeable and not a know it all.
by Rick Warwick August 27, 2008
 
4.
A person who's quick to interrupt the actions, comments, or approach of others on just about everything they say and do, so as to disapprove in some way and try to correct them with their own views on the topic
"Ann: "I think Jerry needs some tylenol and-"

Pat: "No, No, Ann! When someone feels that way, you need to get them some chicken noodle soup."

Elaine: "Well, I'm going to feed the bird so-"

Pat: "NO, NO, ELAINE! For a bird like this, you only feed it three to four times a day and you want to give it honey."

Ray : "I'm going to run out and get some tools to-"

Pat : "Ray, that's rubbish. To fix a bicycle, you have to use old dirty tools. New tools don't get the job done these day. It's the old fashioned kind that will get the job done."

Janice : "Pat, quit it. You're being an motherfucking know it all. Quit it. QUIT IT!"

by Leo Bos July 20, 2008
 
5.
A person who's quick to oppose the opinions, input, or approaches of others in seemingly every aspect of life by always trying to correct others with their own input that they present in an instructive fashion
Ann: "I'll get Jerry some tylenol and-"

Pat: "Actually, when someone feels this way, what they need is chicken noodle soup and crackers."

Amber: "I'll run to the store and-"

Pat: "ACTUALLY making it from scratch is much more healthy than going out to the store and getting it."

Mark : "Pat, quit being a know-it-all. Yo momma!"
by Man Tolam July 20, 2008
 
6.
1. One who's always acting as if things that are abstruse or not necessarily easy to understand are pointedly obvious, and often shows a snobbish hostility towards others if they're not showing complete understanding of it:

someone who treats you like an idiot for not knowing difficult things or acts like difficults things are obvious is a know-it-all

2. One who expresses their unproven opinions and ideas as if they're obvious known facts as if others should have known a long time ago.
"Hahahahaha! Trigonometry. That old stuff. You're having a tough time with that," laughed my know-it-all sister, Megan.

"Well students. You actually have to discern the entomological aspects of it." "Well, DUH! No shit, Sherlock. If you can't, that's pretty sad," said a know-it-all in the class.

"I don't get how to fix the TV," said my father. "Omg! You don't know how to fix a TV set. Lmao! Who doesnt know how to fix a TV set, take it apart and put it back together again. That's so obvious."

Abby said "no duh" to nearly everything she heard during the study on insects. She is such a know-it-all.

"All you do to fix it, is turn it off and then tighten some of the wiring. Duh," said Kya. Kyra had never tried fixing a computer in her life. She was just acting like a know-it-all.

by MikeZatin July 10, 2008
 
7.
One given to showing off about what they know by teaching others on numerous different topics that have little relation or no relation at all to the each other or the main subject matter at hand; pedant
"I'm afraid that's not going to fix the truck, Jim. To fix a truck you have to retool it. Not that much different from fixing a television set in which you rewire the power cords. Power cords which interestingly enough came were invented in the 1800s around the same time jim crow laws were passed and The First Barbary war took place. It's interesting to note that the first law was-" "Can you tell me how to fix the truck or not, know-it-all?" snapped Jim, justifiably so.

My know-it-all father went rambling on about irrelevant shit from how to groom dogs, how to tattoo arms to how to construct a road and how to build a house instead of getting to the point about figuring out the math problem. As it turned out he didn't know how.
by Marvinforbare July 13, 2008