How can "nerve" (n) both mean "courage" but also "nervousness"?

I was looking at the definitions of "nerve" given in Lexico nee ODO, and I got two definitions of it that are quite the opposite of each other:

nerve
noun

1. ...

2. one's steadiness and courage in a demanding situation.
   ^{"an amazing journey which tested her nerves to the full"}

   • ...

3. feelings of nervousness.
   ^{"his first-night nerves soon disappeared"}

4. ...

This pair of definitions and their respective examples above completely oppose each other, methinks.

How does something like this happen? Unless I have misinterpreted these definitions, how can a word have two definitions that make opposing claims to each other?

Thanks :)

Solution 1:

It is a weird idiomatic quirk, but usually, it is the plural "nerves" which is used to describe fear. "Nerve" in the singular usually means "courage":

• "He lost his nerves" - means his fear subsided.
• "He lost his nerve" - means his courage gave way to fear.

Your dictionary quote about "testing nerves to the full" seems like an exception to this, but one explanation may be that it is using "fear" in a positive context, as in "facing your fears". It may also be a true plural of the word "nerve", so "testing your nerves" means it will test your many forms of courage.

Remember that dictionaries collect the singular and plural uses of a word together. Any idiomatic differences are brought out in the multiple definitions and you have to take note of the examples cited.