This is technically French for “great puppet,” but it usually refers to a theater in Paris called Théâtre du Grand Guignol (“The Theater of the Great Puppet.”)
It was known for its realistic, gruesome horror shows. Each performance would be of several short plays.
Here’s a description from a 1957 New York Times article (not linked due to paywall) entitled “Fading Horrors of the Grand Guignol”:
Every night, in an old, somber street off Montmartre, some 250 peaceful citizens are the horrified eyewitnesses of four murders – part of the longest running crime wave in existence. For sixty years now, torrents of blood have flowed, eyes have been gouged, faces singed by fire or disfigured by vitriol, bodies dissolved in acid baths, hands, arms and heads chopped off, and women raped and strangled on the Rue Chaptal. And yet the police have never once thought fit to intervene, for all this gory business takes place on the diminutive stage of the Théâtre du Grand Guignol.
The theater opened in 1897 and closed in 1962.
According to Wikipedia:
Its name is often used as a general term for graphic, amoral horror entertainment…
According to this video about the theater:
Today, the term “Grand Guignol” is synonymous with the “shockers,” the blood – and sometimes even guts – offerings.
Why I Looked It Up
In Slow Horses, regarding a plan to behead a prisoner and broadcast it on the Internet:
Even in the webbed-up world, where conspiracy theories spread faster than a blogger’s acne, PJ had no difficulty believing that elements within MI5 might have concoted this piece of Grand Guignol, and it even impressed him, a bit.