Excruciate (adjective):                                                                                                                                      Causing great pain or anguish : agonizing.                                                                                                  Very intense : extreme

https://www.merriam-webster.com/dictionary/excruciating

From: A Hunger Artist                                                                                                       “Sometimes there were nightly groups of watchers who carried out their vigil very laxly, deliberately sitting together in the distant corner and putting all their attention into playing card there, clearly intending to allow the hunger artist a small refreshment, which according to their way of thinking, he could get from some secret supplies. Nothing more excruciating to hunger artist than such watchers”                                                                                                                               This sentence means that people still thought that hunger artist has some secret ways of having some snakes because people believed no one could bear the hunger for a long time of period. Yet, hunger artist was a true artist who can never break his honesty although people force him to eat during his hunger game. And he was very upset with people who thought he was having secret ways of getting food, which he never did and never will.