"Feigned madness" is a phrase used in popular culture to describe the assumption of a mental disorder for the purposes of evasion, deceit or the diversion of suspicion. In some cases, feigned madness may be a strategy—in the case of court jesters, an institutionalised one—by which a person acquires a privilege to violate taboos on speaking unpleasant, socially unacceptable, or dangerous truths.
Lucius Junius Brutus, who feigned stupidity, causing the Tarquins to underestimate him as a threat until the time when he was able to drive the Roman people to insurrection.
Alhazen, who was ordered by the sixth Fatimid Caliph, al-Hakim, to regulate the flooding of the Nile; he later perceived the insanity and futility of what he was attempting to do and, fearing for his life, feigned madness to avoid the Caliph's wrath. The Caliph, believing him to be insane, placed him under house arrest rather than execute him for failure. Alhazen remained there until the Caliph's death, thereby escaping punishment for his failure to accomplish a task that had been impossible from the beginning.
In 1340, when the French fleet was destroyed at the Battle of Sluys by the English, none of Phillippe VI's courtiers dared to tell him the news. It therefore fell to his jester, in his role as the "fool", to tell him that the English sailors "[didn't] even have the guts to jump into the water like our brave French".
Ion Ferguson, an Irish psychiatrist in the British Army in a WWII German prisoner-of-war camp, successfully feigned madness to get himself repatriated. He also assisted two other prisoners in doing the same.