Thanks for showing interest in Assam.
The word Assam derived from the a-som. The vowel that is referred by 'a' sound(the first letter in assamese alphabet) means negatively, and 'soma' means equal. So a-soma means not equal. Assam as you know is a land that is full of hills, hillocks, rivers and plain lands. So it means that the surface is not a plain land.
From these two words we got the word Assam finally. And this is just an opinion of historians.
RUNA N BORAH