The hard outer covering of a nut. Commonly used in the phrase 'in a nutshell' meaning 'briefly' or 'in summary'.
From Middle English, combining 'nut' and 'shell'. The metaphorical use 'in a nutshell' dates to the 1500s, inspired by the idea of fitting something large into a tiny space.
Shakespeare actually used 'nutshell' metaphorically in Hamlet when the prince says he could be 'bounded in a nutshell' and count himself king of infinite space. This tiny natural container became our go-to metaphor for concise explanation.
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