Solanum carolinense; Solanaceae (nightshade family)
Not really a nettle, despite its common name, but closely related to potato (Solanum tuberosum) and eggplant (S. melogena), and less closely related to tomato (Lycopersicon species), chili pepper (Capsicum species), and tobacco (Nicotiana species). I write this because I can find nothing particularly interesting to say about horse nettle; its inclusion in the Solanaceae is the only thing about it I find remotely interesting. It is an ugly little flower on a weedy-looking, ugly little plant.
Gardeners define a weed as “a plant growing where it isn’t wanted”, so I can’t just call this thing a weed, but according to the USDA, seven US states consider it a “noxious weed”. In Arizona it’s a prohibited noxious weed. Apparently it’s very difficult to remove, as broad-spectrum herbicides have little effect on it, and tend to kill its competition instead. Yay.
Oh, and it’s poisonous.