Although the modern form of Bihu is a synthesis of varied cultural elements from diverse ethnic groups and races like Austro-asiatic, Tibeto-Burman, Aryan and Tai-Shans, it was originally a agricultural festival and came to be known as Bihu after the Ahom Swargadeos patronized it during their historic rule.
The origin of Bihu can be traced to the admix of the cultures of Austric and Tibeto-Burman races which constitute the greater Bodo-kachari race in Assam. However, there are still certain ethnic groups like Khasi, Jaintia in Meghalaya and Keot(Kaibarta) which have Australoid origin.
The word Bihu has been derived from the Deori (a form of Bodo tongue which was once the original language spoken by the Bodo-Kacharis of Upper Assam) word Bisu which means "excessive joy". This form of Bihu was once celebrated by the Bodo-Kachari tribes of Upper Assam and Austric-Dravidian ethnic groups, consisting of Chutias, Sonowal Kacharis,Thengal-Kacharis, Morans, Deoris, Motoks and Keot (Kaibarta) people (majority). Some of these groups were known as Sadiyal Kacharis having lived in the kingdom of Sadiya. The other branch of Bodo-Kacharis which include Bodos, Dimasas, Rabhas, Tiwas, as well as Austro-Dravidian ethnic groups like Keot(Kaibarta) etc have also been celebrating Bihu since ancient times. The Bodos call it Baisagu, while the Dimasas, Tiwa and Rabha call it Bushu, Pisu, Dumsi respectively.It can be understood that over time the word "Bihu" came to replace "Bisu" in the Assamese language as can be seen with many other words like Sap-Hap, Siyal-Hiyal, Sari-Hari, Sewali-Hewali, Siloi-Hiloi, Sahas-Hahokh, etc.
It is also well-known that the modern form of Bihu dance was derived from the Faat Bihu dance celebrated in Dhakuakhana, Lakhimpur. The performers were called by the Ahom king Rudra Singha in 1694 to dance in the royal arena Rang Ghar. The origin of Faat Bihu can be traced to Sadiya. The word Faat in Deori-Chutia language means "to migrate". After the defeat of the Sadiyal Kacharis in Sadiya, the survivors were displaced from Sadiya to different places in the kingdom. A group of these people moved from Sadiya, to Dibrugarh and finally settled down in Harhi Sapori, Dhakuakhana. These people had brought the idols of god and goddess along with them and established a temple now known as Harhi Dewaloi. It was here that the first form of modern Bihu dance was developed. Later, in the 19th century, this form of Bihu dance was adopted by the other communities as well and started being performed in Mahguli sapori, Dhakuakhana by Chutias, Sonowals, Deoris, Ahoms, Mishing, Keots (Kaibartas) etc.
The Tai-Shans/Tai-Ahoms(as called by the natives) upon their arrival in Assam found the earlier settlers(Tibeto-Burmans) celebrating a festival of cow-worship spraying fresh water. This ritual looked similar, to the ancestral Poin-Cham-Nyam ritual of their homeland. So, they called the existing festival of this land Poin-hu which is today known as Bihu. As the earliest mention of Bihu is found in the late medieval history of Assam it is quite evident that the term Bihu originated during the period of Ahoms who were culturally advanced than the other existing communities.
It is believed that the Bihu dance was given royal patronage by the Ahom king Rudra Singha in 1694. The present form of historic singing in Assam was also introduced by the Ahoms. It can be seen that most of the contribution to Bihu was made by the Ahoms.
^ Gogoi, Dinesh(2015), Baapoti Hahun Bihu, Page 17-18, The root of the Bihu festival lies with the earliest immigrants of Assams, the Austro-Dravidian ethnic groups and the Kacharis. It is specially attributed to the Keot (Kaibarta) and the Sadiyal Kacharis which includes Chutias, Sonowals, and Deoris. The word Bihu is probably a derived form of "Bisu" celebrated by the Deoris, which stands for "excessive joy". Other festivals similar to it include the Baisagu of Bodos, Bushu of Dimasas, Pisu of Tiwas, and Dumsi of Rabhas.