the.com/nutshell

means A nutshell is the condensed, bare-bones version of something — the whole story shrunk down to its smallest honest size.

from From 'nut' plus 'shell,' literally the hard little case around a kernel. The figurative sense — 'in a nutshell' meaning summed up briefly — reaches back to ancient legend, retold by Pliny the Elder, that Homer's entire Iliad was once copied out in handwriting small enough to fit inside an actual walnut shell. Whether or not that was ever literally true, the image stuck: something vast, folded down into something you could hold in one hand.