horsechestnut

 The large nutlike seed of a species of Æsculus (Æsculus Hippocastanum), formerly ground, and fed to horses, whence the name. The seed is not considered edible by humans.  [1913 Webster]

 The tree itself (Aesculus hippocastanum), which was brought from Constantinople in the beginning of the sixteenth century, and is now common in the temperate zones of both hemispheres; it has palmate leaves and large clusters of white to red flowers followed by brown shiny inedible seeds. The native American species is also called buckeye and conker.  [1913 Webster]