Plural of 'nerve'; either the physical fibers carrying signals in your body, or emotional boldness and confidence.
From Latin 'nervus' meaning 'sinew' or 'string of a musical instrument.' The metaphorical meaning of 'courage' developed from the physical meaning through the idea of strength and tension.
Calling someone 'nervy' for having courage comes from thinking of nerves like strings on a musical instrument—you need them tight and strong to produce good sound, so having strong nerves meant having strength. This physical-to-emotional metaphor shows how our bodies shape our language.
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