A term meaning "to go to hell in handbasket" or "when the shit hits the fan." Reportedly of British or Cockney origin, from the Royal Air Force's description of circular flight paths gone awry, or "pear-shaped."
Also may relate to a popular French Revolution caricature of "citizen King" Louis-Philippe, drawn by Honore Daumier and Charles Philipon in 1832 as a jowly pear. The French term "poire," or pear, was slang for "fool" or "idiot" - and soon thereafter, all caricatures of the King were banned, pear-shaped or not.
"To say that it all goes pear-shaped is an under statement. The reactors go out of control and quickly the effort is not to save the plant, but to minimise the effects."