An abbreviation for SystemAdministrator.
The term SysAdmin was first used on Multics. 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. You can take a look at the main loop of the Multics answering service. This piece of the Multics system administration code is responsible for logging in users and is roughly equivalent to Unix' getty(8).