I don't think "substitute user" is the original meaning, and it's more like a retroactively applied acronym.

Looking at various old Unix manpages, it said various things in the past. In the HP-UX documentation it even lists three different variants in the same man page: "switch user", "set user" and "superuser".

"superuser" is probably the original meaning, because that's what it says in the Unix Manual 1st edition (1971): http://man.cat-v.org/unix-1st/1/su

NAME	su -- become privileged user
SYNOPSIS	su password
DESCRIPTION	su allows one to become the super--user, who has all sortsof marvelous powers. In order for su to do its magic, the user must pass as an argument a password. If the passwordis correct, su will execute the shell with the UID set to that of the super--user. To restore normal UID privileges,type an end--of--file to the super--user shell

I love Unix archeology :)

You can do su to change the user in the current shell. Afaik it just defaults to root if no user ist specified. Everytime you run su you actually do su root

That said I always thought that it stands for switch user so intereresting to know that it‘s substitute.

I've heard people call it superuser since before Linux (AIX Unix a long time ago for example).

But substitute user makes sense since you can su to any user (just root is the default).