Conkers is a traditional children's game in Great Britain and Ireland played using the seeds of horse chestnut trees—the name 'conker' is also applied to the seed and to the tree itself. Wikipedia

A traditional children's game, using the seed threaded on a string Wikipedia Disambiguation

A game for two players in which the participants each have a horse-chestnut (known as a "conker") suspended from a length of string and take it in turns to strike their opponent's conker with their own with the object of destroying the opponent's conker before their own is destroyed. Wiktionary