Some time ago, I found that IBM Redbooks maintained a dictionary of mainframe jargon that mainframers used.
After learning about the number of acronyms on the #mainframe, I certainly wished I had found that earlier. And, from what I can see, not many people know that the dictionary existed too.
In any case, I gathered a collection of resources that may help you learn more about mainframes. Check it out at https://securitypuppy.com/links.