means A nerve is a fiber that carries signals between the brain and the rest of the body — and, by extension, both raw courage and the brazen audacity to overstep.

from From Latin nervus, meaning 'sinew' or 'tendon' — the tough cords that move the body — which is itself related to Greek neuron, 'sinew' or 'nerve.' For centuries 'nerve' meant literal strength and stringy tissue, which is why a vigorous argument still has 'sinew' and a bold person has 'nerve.' Only later did anatomists narrow it to the body's signal-carrying fibers, while everyday speech kept the older flavor of toughness — and added the cheeky sense of 'the nerve!' for someone with too much of it.