Praise be to Allah.
The Prophet (blessings and peace of Allah be upon him) gave a clear definition of backbiting. He (blessings and peace of Allah be upon him) said: “Do you know what backbiting is?” They said: Allah and His Messenger know best. He said: “When you say about your brother something that he dislikes.” They said: What if what I say about my brother is true? He said: “If it is true then you have backbitten about him and if it is not true then you have told a grave lie about him.”
Narrated by Muslim (2589).
Maalik narrated in al-Muwatta’ (3/150) that a man asked the Messenger of Allah (sa) (blessings and peace of Allah be upon him): What is backbiting? He said: “When you say about your brother something that he does not like to hear.” He said: O Messenger of Allah, even if it is true? The Messenger of Allah (blessings and peace of Allah be upon him) said: “If you say something untrue, then that is a grave lie.”
Classed as saheeh by Shaykh al-Albaani in as-Silsilah as-Saheehah (1992).
Backbiting only refers to speaking ill of a person in his absence, when he is not present. As for criticising him in his presence, this comes under the heading of insulting and reviling, and is also haraam.
The Prophet (blessings and peace of Allah be upon him) said: “Insulting a Muslim is an evil action and fighting him is disbelief (kufr).”
Narrated by al-Bukhaari (48) and Muslim (64).
And he (blessings and peace of Allah be upon him) said: “Do not envy one another, do not artificially inflate prices, do not hate one another, do not turn away from one another, do not undercut one another. Be, O slaves of Allah, brothers. The Muslim is the brother of his fellow-Muslim. He does not wrong him, let him down or despise him. Piety (taqwa) is here” – and he pointed to his chest three times. “It is sufficient evil for a man to despise his Muslim brother. The whole of a Muslim is sacred to another Muslim, his blood, his wealth and his honour.”
Narrated by Muslim (2564).
Al-Haafiz Ibn Hajar (may Allah have mercy on him) said:
Ibn at-Teen said: Backbiting (gheebah) means saying about a man in his absence that which he dislikes. This was also stated by az-Zamakhshari and Abu Nasr al-Qushayri in at-Tafseer, and by Ibn Khamees in a book on this topic, and by al-Mundhiri and more than one of the scholars, one of the last of whom was al-Kirmaani. He said: Backbiting means saying behind a man’s back what he would dislike if he heard it, even if it is true. End quote from Fat-h al-Baari (10/469).
Al-Jirjaani said: Backbiting means saying bad things about a person in his absence, when they are true; if they are not true, then it is a grave lie, and if it is said to his face, then it is reviling or insulting.
End quote from at-Ta‘reefaat (p. 163)
Conclusion: your saying about your Muslim brother that he is boring, meaning that people do not like to sit with him because his character, words and manner are tedious, comes under the heading of backbiting which is haraam, even if it is said in a moment of anger, because anger does not make it permissible to cast aspersions upon your brother who has not wronged you.
There is a stern warning against backbiting that was narrated from the Prophet (blessings and peace of Allah be upon him), so hasten to repent and to apologise to your brother if he has heard about what you said concerning him. If he has not heard about it, then you should seek the forgiveness of Allah, may He be exalted, and pray for your brother.
See also the answers to questions no. 23328 and 6308.
And Allah knows best.