UNKNOWN PLANT

Yes, I'd say it's a weed - it looks like Cow Parsley to me. It grows along the hedgerows in our part of the world!

I think it may be Pig Nut. If the leaves are the same all the way down, then that is likely. Don't eat it though in case it is Fool's Parsley.. If PIg Nut then a nut will be attached to the root, assuming you want to get rid of it. It looks rather pretty to me.

It could be Ammi Majus Kath a annual used quite a lot in flower arranging, I think its quite pretty too.