Baseball terminology. Refers to an upper deck, opposite field home run. Combines two feats of hitting that are hard to achieve to create an extremely rare event. Usually used for intimidation, much like calling your home run before the at-bat.
Batter: Let's go, uppo oppo, baby!
Pitcher: Oh shit, I'm intimidated.
(Insert name of steroid-using slugger) went 4 for 5 last night going long twice and jacking one uppo oppo way into the nosebleed seats.