1. IBM's supremely rude Job Control Language. JCL is the script language used to control the execution of programs in IBM's batch systems. JCL has a very fascist syntax, and some versions will, for example, barf if two spaces appear where it expects one. Most programmers confronted with JCL simply copy a working file (or card deck), changing the file names. Someone who actually understands and generates unique JCL is regarded with the mixed respect one gives to someone who memorizes the phone book. It is reported that hackers at IBM itself sometimes sing “Who's the breeder of the crud that mangles you and me? I-B-M, J-C-L, M-o-u-s-e” to the tune of the Mickey Mouse Club theme to express their opinion of the beast.
2. A comparative for any very rude software that a hacker is expected to use. “That's as bad as JCL.” As with COBOL, JCL is often used as an archetype of ugliness even by those who haven't experienced it. See also IBM, fear and loathing.
A (poorly documented, naturally) shell simulating JCL syntax is available at the Retrocomputing Museum http://www.catb.org/retro/.