My first thoughts were (1) poison ivy, (2) blackberry, and (3) really big mint...but I really have no idea. Are the stalks hairy or have briars on them? It looks like mint in the background, but I've never seen mint get this big. Any smell to the plant if you crush the leaves? (but don't handle them unless you know it isn't poison ivy!).
reminds me of lambsquarter but that's not what it is