$ man su

    NAME    
       su - change user ID or become superuser

It's not quite so simple. The meaning has changed over time.

What coreutils are you using? I get:

  ~$ man su

  NAME
    su - change user ID or become superuser

OS X uses the FreeBSD coreutils and not the GNU ones, so if you're on either OS X or FreeBSD, that would explain it.

This.

I thought it meant super user too, but then I put in that command and said "ah". Then I moved on with my life....