Where do the words in /usr/share/dict/words come from?

[/usr/share/dict/words](https://en.wikipedia.org/wiki/Words_(Unix)) contains lots of words. How is this list generated? Are its contents the same across different Unices? Is there any standard dictating what it must contain? All I've been able to turn up so far is that on Ubuntu/Debian the list comes from the [wordlist](https://packages.debian.org/sid/wordlist) packages, but their descriptions offer no clue on how the lists were actually generated.