A British term for the fruit/seed of a horse chestnut tree. Commonly called a chestnut.
Can also refer to the game called 'conkers' played using the chestnuts in which two opponents attempt to shatter the other's conker. The conkers are both suspended on a length of string and the players take turns to swing at the other's. The player with the conker last remaining on its string is the winner.
Young Boy 1: "Ouch man, you hit my finger!"
Young Boy 2: "Well that's the inherent risk of playing conkers my friend. You know what you signed up for!"