Stem is so thick ( photo / image / picture from Carol D's Garden ) Very fast growing weed! ( photo / image / picture from Carol D's Garden )
Check this one out to see if it might be yours..... Paulownia tomentosa (Empress Tree, Princess Tree)
I don't think it's Paulownia. It kind of reminds me of Phytolacca americana. http://www.gardenstew.com/plantstew/19523