The Collaborative International Dictionary of English v.0.48:

JCL \JCL\, n. [all capitals] (Computers)
   Job Control Language. [Acronym]
   [PJC] JD

V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

JCL
       Job Control Language (IBM, OS/360)


The Jargon File (version 4.4.7, 29 Dec 2003):

JCL
 /J?C?L/, n.

    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/.


The Free On-line Dictionary of Computing (30 December 2018):

Job Control Language
JCL

    (JCL) IBM's supremely rude
   script language, used to control the execution of programs
   in IBM OS/360'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 memorises 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.

   As with COBOL, JCL is often used as an archetype of ugliness
   even by those who haven't experienced it.  However, no
   self-respecting mainframe MVS programmer would admit
   ignorance of JCL.

   See also fear and loathing.

   (1999-03-03)