A nutshell is the outer shell of a nut

Nut (fruit)

A nut is a hard-shelled fruit of some plants having an indehiscent seed. While a wide variety of dried seeds and fruits are called nuts in English, only a certain number of them are considered by biologists to be true nuts...

. Most nutshells are inedible and are removed before eating the nut meat inside.

Usage

Most nutshells are useful to some extent, depending on the circumstances. Walnut shells can be used for cleaning and polishing, as a filler in dynamite

Dynamite

Dynamite is an explosive material based on nitroglycerin, initially using diatomaceous earth , or another absorbent substance such as powdered shells, clay, sawdust, or wood pulp. Dynamites using organic materials such as sawdust are less stable and such use has been generally discontinued...

, and as a paint thickening agent.
Shells from pecans, almonds, Brazil nuts, acorns, and most other nuts (with the exception of peanuts

Peanut

The peanut, or groundnut , is a species in the legume or "bean" family , so it is not a nut. The peanut was probably first cultivated in the valleys of Peru. It is an annual herbaceous plant growing tall...

) are useful in composting.
There is a method for preparing peanuts to be composted, requiring altering the levels of nitrogen

Nitrogen

Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

 and carbon

Carbon

Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

.

Shells can also be used as loose-fill packing material, to protect fragile items in shipping.

Idiomatic usage

The expression "in a nutshell" (of a story, proof, etc.) means "in essence

Essence

In philosophy, essence is the attribute or set of attributes that make an object or substance what it fundamentally is, and which it has by necessity, and without which it loses its identity. Essence is contrasted with accident: a property that the object or substance has contingently, without...

", metaphor

Metaphor

A metaphor is a literary figure of speech that uses an image, story or tangible thing to represent a less tangible thing or some intangible quality or idea; e.g., "Her eyes were glistening jewels." Metaphor may also be used for any rhetorical figures of speech that achieve their effects via...

ically alluding to the fact that the essence of the nut - its edible part - is contained inside its shell. The expression further gave rise to the journalism

Journalism

Journalism is the practice of investigation and reporting of events, issues and trends to a broad audience in a timely fashion. Though there are many variations of journalism, the ideal is to inform the intended audience. Along with covering organizations and institutions such as government and...

 term nut graph, short for nutshell paragraph.

A likely source of the phrase may be found in Shakespeare's Hamlet (Act 2, Scene 2) where the title character exclaims: "O God, I could be bounded in a nutshell, and count my selfe a King of infinite space".

Long before modern electronics, a few scholars made attempts at condensing massive literary works so they could be more easily stored. It became an obsession to some to see just how small they could write. For example, a copy of the Quran was reduced on a parchment measuring four inches by half an inch. These copies were so small it was said they could be stored in a nutshell.