conker
noun
conk·er ˈkäŋ-kər 
1
: a horse chestnut especially when used in conkers
2
conkers plural : a game in which each player swings a horse chestnut on a string to try to break one held by the opponent
Word History
Etymology
conch + -er entry 2, from the original use of a snail shell on a string in the game
First Known Use
circa 1886, in the meaning defined at sense 1