It looks it is a weed, but not so bad. I saw it in an urban area in Europe, Hardiness Zone 8a.
Is it poisonous, or nasty in any other way?
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This is a mallow (Malva), see https://en.wikipedia.org/wiki/Malva. I think Malva sylvestris.
It is not a weed, it was often planted as drug, because it (supposedly) can cure a lot of common illness. In any case, it is also frequent in Europe.