Short for system administrator

SystemsAdministration is what a SysAdmin does.

The term SysAdmin was first used on Multics
* http://www.multicians.org/
It was a mainframe operating system for GE (later Honeywell) mainframes.  It comes from the "project" name used
by system administrators.

The system administration software for Multics was written in about 1969, by Joseph F. Ossanna and Michael J. Spier.  No one remembers now who coined the term or adopted the convention of CamelCase. (The first Multics SysAdmin other than test users was 
TomVanVleck, then a member of the MIT research staff.)

The piece of the Multics system administration code that was responsible for logging in users, roughlyequivalent to Unix getty, is online.
* http://www.multicians.org/dialup_.html