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

Flash Lights Impressively

    (FLI) /FLY/ A joke assembly language
   instruction first documented in the late 1970s in "The Hackers
   Dictionary".

   The FLI instruction was frequently referred to by engineers
   when minicomputers such as the DEC PDP-8, PDP-11 and
   some early microcomputers such as the IMSAI and Altair
   had dozens of front panel lights.

   "When the computer is about to do some long I/O operation,
   stick in a FLI so the accountants won't think the machine has
   hung again."

   (2004-08-23)