The scholars are not unanimous in their opinions about the origin of the generic term 'Mech'. There different schools of scholars give us three different opinions about the origin of the word 'Mech'.
  
The first school of scholars think that the word 'Mech' has been derived from Sankrit word 'Mlechha'.
  
The second school of scholars think the 'Mechha' itself is the Sankritised word of 'Mech'.
  
The thired school of scholars hold entirely different opinion. According to them, neither 'Mech nor 'Mlechha' is a derivation from each other, it is rather an appellation given to the group of people after the name of the river Mechi they were living by.
  
View-I: Mlechha>Mech : Opinions of some of the scholars who think that the appellation 'Mech' has been derived from the Sanskrit word 'Mlechha'.
  
Rev. Endle : The name (Mech) is almost a corruption of the Sanskrit word 'Mlechha', i.e. an outcast from the Brahmin point of view, and non-observer of caste regulation; such person being in the light of modern Hinduism very much what the barbarian was to the Greek......"
  
Farquhar: "The word 'Mech is simply a corruption of the Sankrit word Mlechha which means 'barbarian' or unclean or foreigners."
  
Grierson: The word 'Mech' is probably a corruption of the sankrit word 'Mlechha' which corresponds to the original meaning of the word welsh' i.e. foreigner, stranger."
  
Anderson: "Thename Mech or Mlechha that is 'Welsh' or barbarians, was given to the Bodos of North-Bengal by their Bengali neigbhours."
  
Lebilieh: "There is resemblance between Mlechha and Mech and the Sanskrit Mlechha normally became Prakrit Mlechha or Mechha."
  
N. N. Basu: "The word Mech has been abbreviated from the Sanskrit word 'Mlechha' and that they belong to Asura dynasty."
  
View-2: Mech>Mlechha: The chief exponent of this view is Prof. S. K. Chatterjee who thinks that due to the progressive Sanskritisation of the various Pre-Aryan people in their culture, their outlook and their ways of life, the Mech might have been Sankritised as 'Mlechha."
  
View-3: The Bodos, one of the Tibeto-Burman Speaking Indo-Mongoloid Tribes, migrated into India through Patkoi hills between India and Burma and gradually spread themselves into the whole of modern Assam, North-Bengal. It is probably that they marched towards three directions. One part went along the river Bramaputra and eastablished themselves in the whole of modern Assam upto Goalpara and parst Jalpaiguri and Cooch Behar under the name of Bodo.
  
The third part went towards the west along the foot of the Himalayas upto the river Mechhi between India and Nepal and settled on the North Bank of the said river as Mech or Mechia. Sothe Bodos who settle on the bank of the river Mechi were known as Mech after the name of the river. Then they crossed the riverand established themselves in the deep forest of Darjeeling Terai and Baikuntapur of Jalpaiguri. Again they marched eastwards, crossed the Tista and spread themselves in the Duars, Jalpaiguri. They gradually moved further east, crossed the Sankosh and went towards Goalpara in Assam. The above account in regard to the derivation of the word Mech from the river. Name Mechi which has been stated by Dr. Charu Chandra Sanyal stands in country to the account given by Risley who, when writing about the origin of the 'Mech', says, 'Mech, a Mongoloid Tribe found in the Goalpara district of Assam and in the Himalayan Terai from the Bhutan Duars westward to the Konki river. In Northern Bengal they have given their name to the Mechi river and in Goalpara to the large ract of country called Mechpara.
   
Yet another proposition made by Binoy Khungur Bwisumuthiary in his article "The Origin of the Bara" as regards the origin of the word Mech, which I feel, should not be completely ignored, is that the appellation 'Mech' is the shortened version of Meshek, son of Japhet and grandson of Noah. This highly hypothetical proposition that the modern Meches or Bodos who were one of the constituents of the great Mongol holde are ethnically descendants of Noah (the task of proving Noah's ethnicity is left over to the Anthropologists) leads us refer to yet another hypothetical proposition made by N. N. Basu in his "the Social History of Kamrup" (Valume 1) in which we also get Biblical reference as to the origin of the 'Mech'. N. N. Basu observes, "In the Vedic Age the Asuras became separate from the Aryans and having crossed the borders of India settled in Persia or Turkey. Gradually they incread in strength and influence and it is now over five thousand years since they founded the kingdom of Asur or Assyra about two hundred miles to the North-West of Babylon and founded the capital of Asur on the bank of the Tigris. Their possessions extended from Asia Minor to the Caucasus mountain. In the Old Testament mention is made of a Royal priest under the name of Melchi-zedek :
  
"Then Melchi-zedek king of Salem brought out bread and wine. He was priest of God Most High" (Genesis-14 : 18). From Caucasus he went over to Salem (Jerusalem) and became 'King and Priest of Supreme El'. After his name 'Mekhi' came to denote king and priest' in Syrian and Assyrian languages.
  
It is too well known to be mentioned that in olden times teritoies were generally named after the peoples residing in them. In the Kiskindhya Kanda (43rd chapter) of the Ramayana we are told that far off from India, in the north-west, there lived Mlechhas, the Daradas, the Yavanas etc. And also there was a country named after the Mlechha. From the geographical account given in the epic we have reasons to believe that this Mlechha country was not far from the Mt. Caucasus. It is very likely that it was the priest of this place who were known in ancient times as Melchi (Sanskrit-Mlechha).He further went on to say, "In their zeal to spread and extend their influence the Melchis or Mlechha high priest of Caucasus crossed the Kailasa mountain and reached the hills of Kamrupa via Tibbet. So from the account of N. N. Basu, the derivation of 'Mech' can be shown as Melchi>Mlechha>Mech.
    
One more important proposition which is made on the basis of legends about the origin of the Mech the origin of the Mech is that the appellation 'Mech' is an original designation and not a derivation. Fr. Hermanns has two stories for us. These stories have been recorded by him in his book entitled "The Indo-Tibetans". The first story is found among the Rai people of Nepal. The story goes thus : "Our forefathers came originally from a lake whose waters had dried up. In the centre of Nepal there was a lake which ran dry and dry basin offered a suitable site for settlements. The first parents of the Rai were -Parungo, the father, and Simnia, the mother. They begot three sons-Lapche, Jimdar and Meche. The parents sent the three sons away to live independently and find a country for themselves. The brother Lapche came across a banana tree and tried to hew it down where upon Jimdar said to him, "If you are the kind of person that is prone to be destructive, we shall no longer stay with you; and with this Jimdar and Meche separated and each went his own way. And so in course of time the three tribes Lapche, Jimdar and Meche originated.