to be nosy; to annoyingly look at or comment about someone's food(how the food looks, smells... etc.); to ask who ask someone who they are on the phone with; to look over someone's shoulder when they are reading(books, magazines, computer screen... etc), to ask, comment, or pry into someone's personal life.
This phrase simply means that someone is bugging you about what is going on with you. The phrase is a playful way to tell someone to back off of you and to get out of your business.
This phrase is especially useful in the workplace where many people are in close quarters.
Example 1:
ADAM:(from the next cubicle): Wow... that smells good. What is that? Where did you get that from? Can I have a bite?
MARY: Dude, why are you all up in it?

Example 2:
MARY: Hey, was that really Sally that you just got off of the phone with? Are you still going out with her?
ADAM: Damn! All up in it!
by Carmen November 12, 2004
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(def 1) On top of, either in a literal or metaphorical sense.

(def 2) Pursuing or showing interest in.

(def 3) Imminent or present.

Cultural note: This phrase has received some popularity due to it's occasional use by the character Strong Bad on the Homestar Runner website.
(def 1) George had three puppies, and every time he sat down to watch TV they were all up on his lap.

(def 2) Commercials would have us believe that a guy just has to use the right brand of deodorant and the ladies will be all up ons him.

(def 3) So it's finally Halloween and the Autumn months are all upon us.
by Peter K. September 4, 2007
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Used to mean an encroachment of personal privacy or space, invited or uninvited. Some form of pressure, either physical or mental, i.e. a woman dancing really close to another guy; a landlord pressuring for rent.

Can be used to imply sexual interest, as in someone who is intensely flirting/mackin on someone else, either physically or figuratively. Implies intense or clear sexual interest and pursuit. Same as all over him, related to all up on his balls or all up in his business. Can be used for both genders (i.e. he was all up on her)
Girl was all up on him in the club, backin her booty up.

She was all up on him, wouldn't leave him alone, tryin to get his number n shit

Damn, why you all up on me? I'll get you your shit, I swear.
by ptree March 26, 2008
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the act of satisfying another's deepest needs / physically supplying themselves to stimulate their comfort zones to the point of physical fruition .
One who is "all up in that" is delivering physical pleasure in the deepest and most personal of ways. They are doing so repetitiously and very intently.
by Paul O. Martin August 6, 2006
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Verb. The act of delving fully into an experience or setting, in a deep or otherwise radical fashion.

Noun. The utmost level of commitment to an activity.
"I'm about to get all up in that nash!" Ryan loudly announced as he stepped underneath a 30 foot waterfall.
by MustachedMarvel April 20, 2017
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The state of flirting with a man, especially one who has used the ab-abber. See Homestar Runner.
"The ladies will be all up ons."
by KaiserMonkey August 22, 2003
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Excessively doing something.
A graving or lust.
I was all up in ya mama last night!!
I wanted to put Deez Nutz all up in ya smurf.
by Tadowta April 15, 2006
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