I think it is chicory. If it is, it is a tall, untidy plant but a spectacular blue. Do you live near farmland? It could have come from there.
Now, I am not being insulting, but are you absolutely sure that that is the right leaf and not another plant which is very very close? I think we need an expert on this plant!
Wild chicory has broad lower leaves, as in this photo, but the upper leaves are longer and more pointed.