The original name is not obscure but well known.
The first "su" implementation was written by Dennis Ritchie and Ken Thompson in 1971. From Unix version 1 to 6, this command meaning and purpose was "become super user".
eg: http://minnie.tuhs.org/cgi-bin/utree.pl?file=V5/usr/source/s2/su.c
/* su -- become super-user */
char password[100];
char pwbuf[100];
int ttybuf[3];
main()
{
Its manual page stated:
NAME su -- become privileged user
SYNOPSIS su password
DESCRIPTION su allows one to become the super-user, who has
all sorts of marvelous powers.
However, in version 7, the su command evolved to allow any account to be passed as an option. "su" started then to mean "substitute user".
Version 7 su manual page states
NAME
su - substitute user id temporarily
SYNOPSIS
su [ userid ]
"switch user" appeared only later, mostly in Gnu documents or source code, as a synonym of "substitute user.