A little hard to see for sure, but it looks like an elderberry bush--more like the wild kind, and less like the fancy frilly and colored cultivars. A close-up of where the leaves join the stem should settle the question.

it looks like elderberry to me too and if you don't remember planting it then it is most likely to be a gift from the birds . I'm forever pulling them out of my garden.

I agree looks like a wild elderberry to me too, just pull it out.

Thanks that confirms it as it has gone huge and I knew I hadn't planted it.

Popped up all over Flagstaff, AZ, when I was in college there. Thank you, birds...I think! :)