Abbreviated version of "barking up the wrong tree" which means mistakenly looking for something in a place where it will not be found. Often used to decline a general invitation before it can be specifically directed towards you.
Person A: "I heard the cops were searching your brother's house for evidence."
Person B: "Yeah, wrong tree there."
(Person A approaches two of her roommates)
Person A: "Oh, I'm so glad I ran into you guys! I really need some help cleaning the apartment."
Person B: "Wrong tree."