Expressed very concisely; summarized in the briefest possible way.
This phrase comes from an ancient story told by Pliny the Elder about Homer's 'Iliad' being written in such tiny script that it could fit inside a walnut shell. The story was likely apocryphal, but it captured the imagination as the ultimate example of compression. Shakespeare used the phrase in 'Hamlet' (1602), and it became common by the 1600s.
The phrase taps into our fascination with miniaturization - from ship-in-a-bottle crafts to modern microchips. What's delightful is that we still use this ancient metaphor in our age of digital compression, where we actually can fit entire libraries in spaces smaller than nutshells, yet the phrase retains its charm and clarity.
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