Source : Free On-Line Dictionary of Computing

JFCL
     
        /jif'kl/, /jaf'kl/, /j*-fi'kl/ (obsolete) To cancel or annul
        something.  "Why don't you jfcl that out?"  The fastest
        do-nothing instruction on older models of the {PDP-10}
        happened to be JFCL, which stands for "Jump if Flag set and
        then CLear the flag"; this does something useful, but is a
        very fast no-operation if no flag is specified.  Geoff
        Goodfellow, one of the jargon-1 co-authors, had JFCL on the
        licence plate of his BMW for years.  Usage: rare except among
        old-time PDP-10 hackers.
     
        [{Jargon File}]
     
        (1994-11-22)