In a nutshell
In a few words; concisely stated.
The meaning of the phrase 'in a nutshell' is fairly easy to deduce. Anything that could be written in so few words that it would fit into a nutshell would have to be brief and to the point.
The first text that was supposed to be enclosed in a nutshell was far from small. Pliny the Elder recorded an event, which he apparently believed to be genuine, in Natural History, the original of which was written in AD 77 and was translated into English in 1601 by Philemon Holland, who included explanatory notes, like this:
We find in Histories almost incredible examples of sharpness of the eyes. Cicero hath recorded, that the poem of Homer called the Iliad, written on parchment, was enclosed within a nutshell. The same writer maketh mention of one who could see to the distance of 135 Miles.
Almost incredible? The Iliad is about 700 pages of A4 text and in Homer's day would have been written on clay tablets. This story seems to be an early example of Chinese whispers, long before that term was invented. Holland was translating a piece by the Roman author Pliny the Elder, who had been told by someone that Cicero had seen a minutely written version of a text by the Greek author Homer.
Shakespeare, who often took themes from the classics, alluded to the 'something compact' idea of 'nutshell' when he gave Hamlet the line:
I could be bounded in a nutshell, and count myself a king of infinite space, were it not that I have bad dreams.
The figurative use of 'in a nutshell' to mean specifically 'in few concise words' didn't emerge until the 19th century. Thackeray used it in print in The Second Funeral of Napoleon, 1841:
Here, then, in a nutshell, you have the whole matter.
Squeezing books as long as the Iliad into a nutshell was beyond the capability of an ancient Greek with a stylus, but modern nanotechnology has made it straightforward. The Bible seems to be the book that people favour these days for microminiaturization. Many versions exist that would fit inside a walnut shell, and some readable (with a microscope) texts can fit on a pinhead.