Understand 'nutshell' as an actual shell of a nut - nothing symbolic or abstract. As a child, he collected nutshells to study their different shapes and textures.
Pro Tip 2/3
Natural context
Use 'nutshell' to discuss subjects related to botany or when describing the physical characteristics of nuts. The squirrel gnawed through the nutshell to reach the nutritious seed inside.
Pro Tip 3/3
Not a container
'Nutshell' is not used for containers other than natural ones that encase nuts. A seashell may look similar to a nutshell, but it's not the same thing.