See here from the guy who invented it:
https://hackaday.com/2014/05/28/interview-inventing-the-unix...
and here
https://www.youtube.com/watch?v=LaAwl3HN5ds (from around 4:30 minutes... "do a superuser thing")
Also the "su" command originally stood for "superuser":
https://web.archive.org/web/20220317213155/https://www.bell-... (UNIX manual from 1971)
"su allows one to become the super—user, who has all sorts of marvelous powers"