When we look at the word nervous, we immediately see that root word of nerves in there, so it’s no surprise to learn that the original, scientific meaning of the word was simply “something that contains nerves, affecting the sinews.” It comes via the Latin nervus, which meant “sinew, nerve.” Nervous began being used in that technical sense in the 1300s, and by the 1660s, it was used for anything “belonging to the nerves” as well.
Here’s the interesting bit though. Starting around 1630, it began to take on a metaphorical sense…but definitely NOT the one we’re familiar with! It was used to mean “showing vigor of mind, characterized by force or strength,” and was frequently used in reference to things like writing styles or energetic performances.
So when did our modern meaning begin to appear? Well, by the 1730s, the opposite meaning from the one above–“suffering a disorder of the nervous system”–had entered the language. And from there, it only took a decade for our familiar “restless, agitated, lacking nerve, weak, timid, easily agitated” definition to be established.
Interestingly, this meaning became so popular and so common that scientists were forced to coin the term neurological to differentiate actual “nervous system” meanings from the new definition!
Nervous wreck appeared as a phrase in the 1860s, as did nervous breakdown.