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.


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

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.