nut

An indehiscent fruit having a single seed enclosed in a hard shell, such as an acorn or hazelnut.

The usually edible seed of such a fruit.

Any of various other usually edible seeds enclosed in a hard covering such as a seed coat or the stone of a drupe, as in a pine nut, peanut, almond, or walnut.

A crazy or eccentric person.

An enthusiast; a buff.

A difficult endeavor or problem.

The human head.