I believe it's 'wild parsnip', that's what my dad always called it. (Edit: scientific name "pastinaca sativa"). Google 'wild parsnip' and it will show you some pics at the top of the page. The juice from this plant can cause you to break out in blisters. When my cousin and I were younger we were crawling through the weeds playing soldier or something. The next day we had big streaks of large liquid filled blisters on our arms. My grandpa used some old stuff he called 'copperas' or something like that to dry them up. No telling what it was, but it worked!
Edit: Not that it's that important, but I did a search on copperas and found the proper name is ferrous sulphate. It's not exactly something most people would use nowadays, but it worked with no ill side effects. And besides, the extra thumbs my cousin and I have growing out of our arms can easily be covered with a long sleeve shirt!