dated A euphemism for particularly thick fog or smog. Originally used to refer to the thick, brown, sometimes lethal fog caused by air pollution in London during the 19th and early 20th century, humorously likened to the thick soup made from split peas. For many older citizens living London during that time, they could as easily be killed by a pea souper as by an attack from a criminal. Heathrow had to suspend all outgoing flights for nearly two hours on Sunday due to a particularly bad pea souper hanging over the city.