Sysop (pronounced sis-op) is the short name for System Operator. The term sysop was originally used for mainframe computer operators and predates the term System Administrator or sysadmin which is used today. The term sysop was made popular by the BBS scene of the 1980’s and early 90’s.
If you used BBS’s or you’re curious about the history of the BBS, I highly recommend the BBS Documentary.