The Meaning of Mistrust
Su’ linguistically: sa’ahu yasu’uhu saw’an wa sawa’an: he has afflicted another with what he would dislike. It is the opposite of sarrah (he brought him joy).  Lisan al-`Arab, Ibn Manzur (1/95-96), with minor adaptation.
Zann linguistically: zanantu dhalika wa zanantuhu zannan (I believe such-and-such of so-and-so) is said if one accuses another. Zinnah is tuhmah (accusation). Zanin is the accused, he who is thought to have enacted some accusation.  The previous source (13/273).
Su’ al-Zann technically: is distrusting those around one who are deserving of trust. It is also described as a form of undeserving accusation and betrayal to family, close ones, and all people.  Adab al-Dunya wa al-Din, al-Mawardi (1/186); Tafsir Ibn Kathir (7/377). The Difference between Mistrust and Some Other Characteristics  Qut al-Qulub, Abu Talib al-Makki (2/371); al-Ruh, Ibn al-Qayyim (1/237-238), with adaptation.
● The Difference between Mistrust and Wariness (Ihtiraz)
Wariness manifests with preparedness, care, and taking means to get out of whatever expected harm there may be. As for mistrust, then it is the heart filling up with evil thoughts about people until they overflow onto one’s tongue and limbs. They are always with him in his hurtful speech, throwing around insults, blame, and hatred.
● The Difference between Mistrust and Acumen (Firasah)
Acumen is what one judges about his brother based on some proof that has become noticeable to one, or some evidence, or a sign that hints at something, such that one’s acumen makes a judgement off of it. If this judgement is unfavourable, it is not spoken of. There is no verbal chastisement nor clear audible judgement made, incurring sin. Mistrust is what is thought of a brother due to one’s own bad opinion, or due to some malice, evil intention, or malignancy one holds. A person knows some evil trait in himself and judges others based on his own faults in analogy. This is mistrust and sinful suspicion.Dispraise of Mistrust and its Prohibition in the Qur’an and Sunnah
❖ Allah, exalted, says: “Believers, avoid making too many assumptions - some assumptions are sinful - and do not spy on one another or speak ill of people behind their backs: would any of you like to eat the flesh of your dead brother? No, you would hate it. So be mindful of Allah: Allah is ever relenting, most merciful.” al-Hujurat: 12.
❖ Abu Hurayrah, Allah be pleased with him, narrated that the Prophet ﷺ said: “Beware of supposition, for it is the most false of speech.”  Reported by al-Bukhari (5143) and Muslim (2563). Quotes of the Predecessors and Scholars on Dispraising Mistrust  See: Ihya’ `Ulum al-Din, al-Ghazali (2/177); Rabi` al-Abrar wa Nusus al-Akhyar, al-Zamakhshari (3/257); `Umdat al-Qari, al-`Ayni (23/232).
❖ `Ali ibn Abi Talib, Allah be pleased with him, said: “If people are well over a good period of time, then a man has mistrust of another who never manifested anything disgraceful, then he is unjust.”
❖ al-Ghazali said: “Having mistrust is the backbiting of the heart.”
❖ al-Khattabi said: “False beliefs are the generators of most lies.”Negative Impacts of Mistrust
A cause for falling into polytheism, innovation, and misguidance. It is a trait of those whom Allah does not love. A reason to deserve Allah’s curse and His wrath. It begets evil morals. Whoever has evil suspicions will have evil deeds. It is a cause of grudges and hostility. Forms of Mistrust
Mistrust has many forms in various categories that are impossible to enumerate. Everyone - but those Allah shows mercy - will have their share of it.
Ibn al-Qayyim says: “Most of creation - rather all of them, save whom Allah wills - think of Allah other than what is true, having poor opinions of Him. Most of the children of Adam think that they are missing their full rights, unfairly treated, and deserve better than what Allah has given them. His state bespeaks what his self testifies to, saying: ‘My Lord oppresses me and prevents what I deserve!’ though his tongue denies it, and does not state it explicitly. Whoever searches deep in his self, knowing its innermost buried inhibitions will find this within it; just like fire is dormant within the lighter. Strike the trigger of anyone’s lighter and its sparks will tell you of what lies dormant within. If you pick the mind of anyone, you will find that they withhold judgement against decree, blame for it, and suggestions to what it ought to instead have been - ‘such and such should have happened.’ They either have this abundantly or a little. Search your own self, and find out whether you are safe from this… let the smart one seeking betterment take care of this matter.”  Zad al-Ma`ad fi Hady Khayr al-`Ibad, Ibn al-Qayyim (3/211). Reasons for Falling into Mistrust  Rawdat al-`Uqala’, Ibn Hiban al-Busti (1/100); Qut al-Qulub, Abu Talib al-Makki (2/371); Ihya’ `Ulum al-Din, Abu Hamid al-Ghazali (2/119); Zahirat al-Ghuluw fi al-Din fi al-`Asr al-Hadith, Muhammad `Abd al-Hakim (1/201-202).
Ignorance, bad intentions, and misunderstandings. Following one’s whimsical desires, placing blanket judgements on people. Accompanying the lewd and the wicked. Being present at times of potential lack of clarity and doubt. Malice and envy against the person one has suspicion of. Being excessively jealous and overprotective. Means to Abandoning Mistrust  Zad al-Ma`ad fi Hady Khayr al-`Ibad, Ibn al-Qayyim (3/206-211).
Seeking refuge in Allah, desisting from letting one’s thoughts run unrestricted. Knowing Allah’s Names and Attributes according to how the Predecessors understood them. Fearing the punishment for the one who has evil suspicions. Having a bad opinion of one's self, accusing it of shortcomings. Always taking the self to account, seeking forgiveness. Knowing the ruling of having evil suspicions of Muslims. Avoiding seeking clarification in bad suspicion. Mistrust in Poetry
Abu al-Tayyib said:
“If one’s deeds are evil, so will be his suspicions,
And he will believe what he normally brushes off.
He has enmity to his beloved for what enemies say,
Waking to a wretchedly dark night, full of uncertainty.”  al-`Uzlah, al-Khattabi (p. 31).