conkers (n.)

"child's game played with horse chestnuts," originally with snail shells, 1876, probably a variant of conquer. The goal was to break the other player's item.

In the boy's game of conkers the apexes of two shells are pressed together until one is broken, the owner of the other being the victor. [C. Clough Robinson, "A Glossary of Words Pertaining to the Dialect of Mid-Yorkshire, 1876]