It’s a horse chestnut, otherwise called a ‘conker’.As kids, we used to climb the trees in the fall and shake all the conkers out. Then you drill a hole through the middle and thread a shoelace through, tie a knot in the lace so that the conker sits on the knot.
You then smash your conker against other kids ones, taking turn until one smashes. The winning conker keeps adding to it’s number of conquests, so if it has won 6 battles in a row it is a ‘sixer’. If a sixer fights a fiver the winning conker will be an ‘elevener’ and so on. Some kids used to cheat and inject rock hard glue inside the conker to make it more durable. Now we have the internet, and here I am explaining conkers.