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

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)