Ferlus (2006c: 107-17) goes a step further in assuming that the terms Siam, Assam/ Ahom, Shan originate in a phonological truncation of Sanskrit (ko)sam(bi) > syam/syam in Old Burmese and syam in Angkorian Khmer and Old Cham.
THE SHAN FROM UPPER ASSAM: THE ETHNONYMS TAI AHOM, TAI AITON, TAI KHAMTI
Though the Tai Ahom appeared as early as the thirteenth century in Upper Assam, the others arrived much later, around the eighteenth to nineteenth centuries.
If we may rely on the Buranji (Tai Ahom annals), a Tai Maw prince named Sukapha would have left his homeland around 1215 A.
Around the eighteenth century, (45) while the Tai Ahom were declining, other Thai ethnic groups arrived In Assam from Burma.
It should be noted that it is very probable that contacts never really stopped between the Tai Ahom and the other Shan ethnic groups from Southeast Asia.
Morey (2005: 26), the autonym for the Tai Ahom as found in old manuscripts is simply Tai [[taj.
Diller (1992: 6-7) assigns to the ethnic term Ahom two distinct senses.
To summarize, the ethnonym Ahom is nothing but an "assamization" of the exonym Siam, or Shan in its Burmese pronunciation, a widespread exonym among the peoples of Peninsular Southeast Asia.