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)
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.
(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 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.
spearker 2: Dude, I know. It must have been that hot shirt you were wearing.