Hard to tell by the picture, but looks like Jimson weed to me. Leaves will resemble stinging nettle. I always heard Jimson was poisonous too. A lavender/light blue/whiteish flower on each stalk branch before the pods start developing.
I have one small area by my loafing shed that a plant or 2 comes back from the first year I had one appear and didn't get it cut down or sprayed early.
Inside each of those spiny ball looking seed pods are about 100 little bitty black seeds.You'll have a bumper crop next year. 2,4d will kill it in early growth stage.
other names for it:
Doesn't look like Jimson weed. Jimson doesn't get those points under the seed head. You're right about it being poisonous,GB. At one time, kids were pulling the seed pods, boiling them and drinking the juice. Killed 'em.