Substitute-User or Super-User ?

[dan cornett]

The "su" does not stand for super-user; this meaning is given it by those
who do not have experience with UNIX. "substitute user" is close to the
meaning: "set user" or "set userid" (I saw both used in the earliy 80's,
but I no longer have the old man pages, so I have forgotten which was the
"official" meaning.)

The optional userid argument is of main use to root.  Root can su to any
other user without knowing the password; this facilitates helping other
users when the system operator is working as root and one of the system
users says "such and such isn't working in my account."

Side note: this is why root and system operators should make "." the last
directory searched (or better, do not include "." in the search path). su has
been the source of many Trojan horse attacks by users who ask for "help"
which requires an su.

Danny