The Meaning of Foolishness and Stupidity
Safah linguistically: is the opposite of forbearance. Its origin is lightness and movement. Tasaffahtu fulanan `an malih is said to mean that one has tricked another out of his wealth.  al-Sihah, al-Jawhari (6/2234-2235).
Safah technically: is the opposite of forbearance. It is swift anger, recklessness due to minor issues, instigating fights, excess in harming one’s foe, extravagance in punishment, intolerance to the smallest of afflictions, and cursing and obscenity.  Tahdhib al-Akhlaq, al-Jahiz (p. 29).
Humq linguistically: is a lack of intellect.  al-Sihah, al-Jawhari (4/1464-1465).
Humq technically: is putting a thing in other than its rightful place whilst realising the incongruence.  al-Nihayah, Ibn al-Athir (1/442). The Difference between Foolishness and Stupidity and Some Other Characteristics  al-Furuq al-Lughawiyyah, al-`Askari (p. 278); Tashil al-Nazar, al-Mawardi (p. 5), al-Kulliyyat, al-Kafawi (p. 1021).
● The Difference between Stupidity and Ignorance (Jahl)
There are two facets to this: the first is that the stupid is he who rationalises the irrational, while the ignorant is the one who cannot distinguish between them. The second is that the stupid one is he who knows what is right but does not do it, while the ignorant does not know what is right. If he did, he would do it.
● The Difference between Foolishness and Recklessness (Taysh)
Foolishness is the opposite of wisdom, and it is used figuratively to refer to ugly speech. It is said: safiha `alayh to mean he allowed another to hear from him what is ugly. The ignorant is also referred to as foolish. Recklessness is lightness accompanied by wrong action.Dispraise of Foolishness and Stupidity and their Prohibition in the Qur’an and Sunnah
❖ Allah, exalted, says: “If the debtor is feeble-minded, weak, or unable to dictate, then let his guardian dictate justly.” al-Baqarah: 282.
❖ He, exalted, also says: “Lost indeed are those who kill their own children out of folly.” al-An`am: 140.
❖ Abu Hurayrah, Allah be pleased with him, narrated that the Messenger of Allah ﷺ said: “’The people will be overtaken by years of treachery: the liar will be regarded as honest, and the honest man will be regarded as a liar; the traitor will be regarded as faithful, and the faithful man will be regarded as a traitor; and the Ruwaybidah will have their voice heard.’ It was said: ‘Who are the Ruwaybidah?’ He said: ‘Foolish men who speak about the affairs of the people.’”Quotes of the Predecessors and Scholars on Foolishness and Stupidity
❖ Mujahid said: “I was with Ibn `Abbas when a man came to him and said that he divorced his wife by three pronouncements. He kept silent until I thought that he was going to restore her to him. He then said: “A man goes and does a stupid thing and then says ‘Ibn `Abbas! Allah has said: “Those who are mindful of Allah, He makes a way out for them.”’ al-Talaq: 2. You were not mindful of Allah, and I do not find a way out for you. You disobeyed your Lord and your wife is separated from you.”  Reported by Abu Dawud (2197) and al-Bayhaqi (15338). Ibn Hajar authenticated its chain in Fath al-Bari (9/275). al-Albani authenticated it in Sahih Sunan Abi Dawud (2197).
❖ Yusayr ibn `Amr said - and he had met some of the Companions -: “Abandon the stupid, for there is nothing better for him than his abandonment.”  Reported by Ibn Hibban in Rawdat al-Uqala’ (p. 118). Categories of Foolishness
Foolishness with worldly matters, like dealing with matters with wastefulness and extravagance. Allah, exalted, says: “Do not entrust your wealth to the feeble-minded.” al-Nisa’: 5. Foolishness in religion. Allah, exalted, says: “They are the fools.” al-Baqarah: 13.; “The foolish people will say: ‘What has turned them away from the prayer direction they used to face?’” al-Baqarah: 142. Those intended by ‘the foolish’ here are the disbelievers and hypocrites. Allah, sanctified, says quoting the jinn: “The foolish of us used to utter outrageous falsehoods about Allah.” al-Jinn: 4. Signs of Foolishness
Avarice. Bad mannerisms. Excessively asking the people.  Majma` al-Amthal, al-Nisapuri (2/459). Obscenity and having a filthy tongue. Signs of Stupidity
Hastiness, light-mindedness, contempt, delusion, and wickedness. Ignorance, loitering, betrayal, injustice, and being lost. Delinquency and heedlessness. Haughtiness, evil, and slyness. Impacts and Harms of Stupidity
The stupid: Thinks he is the most intelligent of men. Hates people. Is unknown of in this life. Is not praised nor accepted by Allah or the righteous.  See: Rawdat al-`Uqala’, Ibn Hibban (p. 123). Performs worship only out of habit.  Miftah Dar al-Sa`adah, Ibn al-Qayyim (1/164). Has an incurable disease.  Rawdat al-`Uqala’, Ibn Hibban (p. 202). And the foolish have poor manners and morals. And the foolish are indecent.
The foolish: Gets involved in backbiting people’s respect and honour. Has a filthy tongue. Reasons for Falling into Foolishness and Stupidity
Arrogance and self-amazement. Ignorance of one’s faults. Beguilement by wealth. Bad mannerisms and morals. Means to Desisting from Foolishness and Stupidity
Adorning oneself with forbearance. Humility and brokenness. Moderation and deliberation in matters: Deliberation in obligatory matters. Not laughing excessively. Not mixing with evil people. Respecting good people.  Rawdat al-`Uqala’, Ibn Hibban (p. 119), with minor adaptation. Adages, Proverbs, and Poetry about Foolishness and Stupidity  al-Amthal Ibn Salam (p.125).
❖ “Man’s enemy is his stupidity, and his friend is his intellect.”
❖ “Having animosity to the intelligent is better than befriending the stupid.”
❖ Wahb said: “Wisdom has within it inscribed: ‘The fruit (qasr) of foolishness is tiredness. The fruit of forbearance is restfulness. The fruit of patience is gain.’ The qasr of a thing and its quasar is its aim or fruits.”  `Idat al-Sabirin, Ibn al-Qayyim (p. 95).
❖ Di`bil al-Khuza`i said:
“Enmity with the rational is better to
Have than befriending the stupid.
The rational, if he strays away from his
Forbearance, has shame so rebels not.
You will not find the stupid constant over
Religion, love, nor is he wary of anything.”  Sayd al-Afkar, al-Qadi al-Mahdi (1/60).