Nasb in Arabic refers to setting something up or raising it. Mukhtaar al-Sihaah, 1/275.
In al-Qaamoos it says that the Naasibis (al-nawaasib, al-naasibah and ahl al-nasb) are those whose religious beliefs include hating ‘Ali (may Allaah be pleased with him) because they set themselves up against him, i.e. took a hostile stance against him.
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said: The Naasibis are those who set themselves up against Ahl al-Bayt (the members of the Prophet’s household) and hated them and slandered them. They are diametrically opposed to the Raafidis (Shi’ah). Sharh al-Waasitiyyah, 2/283.
Shaykh al-Islam Ibn Taymiyah said, explaining the ‘aqeedah of Ahl al-Sunnah wa’l-Jamaa’ah: They (i.e., the Sunnis) love the people of the household of the Messenger of Allaah (peace and blessings of Allaah be upon him); they regard them with love and loyalty, and they heed the command of the Messenger of Allaah (peace and blessings of Allaah be upon him) concerning them… but they reject the way of the Raafidis who hate the Sahaabah and slander them, and they reject the way of the Naasibis who insult Ahl al-Bayt in words and deed. Ahl al-Sunnah do not indulge in discussions about the disputes that took place among the Sahaabah.
Al-‘Aqeedah al-Waasitiyyah, Majmoo’ al-Fataawa, 3/154.
So the Naasibis are those who hate Ahl al-Bayt, especially ‘Ali (may Allaah be pleased with him), and some of them slander him and some accuse him of being a rebellious evildoer, and some of them regard him as a kaafir, as was referred to by Shaykh al-Islam (Manhaaj al-Sunnah, 7/339).
One of the most well-known groups who emerged from among the Naasibis were the Khaarijis who rebelled against ‘Ali (may Allaah be pleased with him) and regarded him as a kaafir, and they added other innovations to that.
Undoubtedly rebelling and hating the Ahl al-Bayt and other Sahaabah is a serious kind of bid’ah (innovation) that implies slandering this religion which was transmitted to us via the Sahaabah, the Ahl al-Bayt and others.
With regard to whether they are to be regarded as kaafirs, this varies according to the level of hatred for the Sahaabah, and their motives. In brief, if they hate them for some worldly reason then that does not mean that they are kaafirs or hypocrites, but if it is for a religious reason, because they are the companions of the Messenger of Allaah (peace and blessings of Allaah be upon him), then this is kufr. Anything in between that is an area of scholarly dispute in general. See question no;45563
With regard to the ruling on the Khaarijis – who have a similar mentality to the Shi’ah and added to that hatred of the Sahaabah, regarding the one who commits a major sin as a kaafir, and other kinds of bid’ah – there is some difference of opinion among the scholars. Shaykh al-Islam Ibn Taymiyah said:
The ummah is agreed on condemning of the Khaarijis and regarding them as having gone astray, but they dispute as to whether they are to be regarded as kaafirs. There are two well-known views in the Madhhabs of Maalik and Ahmad. In the madhhab of al-Shaafa’i there is also a dispute as to whether they are kaafirs. Hence there are two views in the madhhab of Ahmad.
The first is that they are wrongdoers, and the second is that they are kaafirs like the apostates, so it is permissible to kill them first, to kill those taken prisoner, and to pursue those who run away. If possible they should be asked to repent as in the case of apostates: if they repent all well and good, otherwise they are to be executed.
Majmoo’ al-Fataawa, 28/518.
Shaykh al-Islam Ibn Taymiyah said: With regard to viewing them as kaafirs and stating that they will abide in Hell for eternity, there are also two well-known scholarly views, which were both narrated from Ahmad. The two views apply to the Khaarijis and those who went astray such as the Harooris, Raafidis and others. The most correct of these views is that their beliefs which are well known to go against what the Messenger brought constitute kufr. Similarly their actions which are like the actions of the kuffaar against the Muslims are also kufr. I have mentioned the evidence for that elsewhere. But to declare a specific individual among them to be a kaafir and to judge that he will abide in Hell forever is dependent upon the conditions for declaring a person to be a kaafir being met and the impediments to so doing being absent.
When we quote the verses and reports which speak of promises and warnings, and who is a kaafir and a faasiq, we should quote them in a general sense. We cannot judge that any specific individual is included in the general meaning of those texts, unless one of the conditions is met with no impediment. We have already discussed this principle in Qaa’idat al-Takfeer. (Majmoo’ al-Fataawa, 28/500). And Allaah knows best.
The questioner should note that in their books the Raafidis who go to extremes with regard to ‘Ali and the Ahl al-Bayt, and slander the Sahaabah and regard them as kaafirs, often accuse those who disagreed with their falsehood of being Naasibis, but by Naasibis they mean Ahl al-Sunnah wa’l-Jamaa’ah. They do this to express their dislike of them for going against their falsehood and following the way of truth. Shaykh al-Islam Ibn Taymiyah said: “With regard to Ahl al-Sunnah, they regard as friends all the believers. When they speak it is on the basis of knowledge and fairness, unlike those who are ignorant or follow their whims and desires; they reject the way of both the Raafidis and the Naasibis and they hold all of the early generations in high esteem, and they recognize status and virtue of the Sahaabah and respect the rights of Ahl al-Bayt as prescribed by Allaah. They also recognize the varying status of members of the early generation, and they recognize that Abu Bakr and ‘Umar enjoyed precedence and had virtues that were not shared by anyone else among the Sahaabah. Manhaaj al-Sunnah, 2/71
Based on this, we have to know who uses this word and who they are referring to by that, lest we reject the truth, because of their describing the people of truth in an incorrect manner. Because one of the characteristics of the people who follow innovation is to attack Ahl al-Sunnah and describe them in offputting terms. What counts is that which is in accordance with the Qur’aan and Sunnah and the way of the earliest generations of this ummah, no matter how much the followers of falsehood try to distort it.
Among the books which speak of the Naasibis and refute them and their ideas, and discussed those who went to the other extreme, namely the Raafidis, is Manhaaj al-Sunnah by Shaykh al-Islam Ibn Taymiyah. You can refer to this book or some of its abridged editions.
We ask Allaah to guide us and you to follow His Book and the Sunnah of His Prophet (peace and blessings of Allaah be upon him), and to protect us from misguidance and temptations both obvious and subtle.