The Free On-line Dictionary of Computing (19 January 2023):

smoking clover

   [ITS] A display hack originally due to Bill Gosper.  Many
   convergent lines are drawn on a colour monitor in AOS mode
   (so that every pixel struck has its colour incremented).  The
   lines all have one endpoint in the middle of the screen; the
   other endpoints are spaced one pixel apart around the
   perimeter of a large square.  The colour map is then
   repeatedly rotated.  This results in a striking, rainbow-hued,
   shimmering four-leaf clover.  Gosper joked about keeping it
   hidden from the FDA (the US's Food and Drug Administration)
   lest its hallucinogenic properties cause it to be banned.


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

smoking clover
 n.

    [ITS] A display hack originally due to Bill Gosper. Many convergent lines
    are drawn on a color monitor in such a way that every pixel struck has its
    color incremented. The lines all have one endpoint in the middle of the
    screen; the other endpoints are spaced one pixel apart around the perimeter
    of a large square. The color map is then repeatedly rotated. This results
    in a striking, rainbow-hued, shimmering four-leaf clover. Gosper joked
    about keeping it hidden from the FDA (the U.S.'s Food and Drug
    Administration) lest its hallucinogenic properties cause it to be banned.