On the ITS system there was a program that allowed you to see what was being printed on someone else's terminal. It spied on the other guy's output by examining the insides of the monitor system. The output spy program was called OS. Throughout the rest of the computer science (and at IBM too) OS means `operating system', but among old-time ITS hackers it almost always meant `output spy'.
OS could work because ITS purposely had very little in the way of `protection' that prevented one user from trespassing on another's areas. Fair is fair, however. There was another program that would automatically notify you if anyone started to spy on your output. It worked in exactly the same way, by looking at the insides of the operating system to see if anyone else was looking at the insides that had to do with your output. This `counterspy' program was called JEDGAR (a six-letterism pronounced as two syllables: /jed'gr/), in honor of the former head of the FBI.
But there's more. JEDGAR would ask the user for `license to kill'. If the user said yes, then JEDGAR would actually gun the job of the luser who was spying. Unfortunately, people found that this made life too violent, especially when tourists learned about it. One of the systems hackers solved the problem by replacing JEDGAR with another program that only pretended to do its job. It took a long time to do this, because every copy of JEDGAR had to be patched. To this day no one knows how many people never figured out that JEDGAR had been defanged.
Interestingly, there is still a security module named JEDGAR alive as of late 1994 -- in the Unisys MCP for large systems. It is unknown to us whether the name is tribute or independent invention.