nice is a program found on Unix and Unix-like operating systems such as Linux....

nice is a program found on Unix and Unix-like operating systems such as Linux....
A niceness of −20 is the highest priority and 19 is the lowest priority.

I was wondering for years why the higher the priority is, the lower the "nice number" is.

Turns out that when the "nice number" of a process is high, it means the process is "nice" to
other processes, i.e. it lets other processes run more importantly than itself.