I have posted so often on the Buckeye Tree in my front yard so I thought it may be an easy guess. I have two things in my front yard. The Ohio Buckeye Tree (a deciduous tree from the Horse Chestnut family) and three varieties of squirrels, Black, Grey and Red (Fox). The squirrels go into the Buckeye Tree in the fall and bury the nuts from the tree to dig up during the Fall and Early Winter. The ones they forget become little Buckeye Trees the next year. I cut at least 30 trees down when I cut my grass this year. The Ohio Buckeye Tree has a yellow flower and the shell of the nut is brown with some small spikes that don't hurt when touched.
So my little plant was not a weed, but a new sapling from a Buckeye a squirrel planted last year. Here is the progression of the buds this spring.
The Buckeye Tree is blooming now and the bees are working on the tree.