Tolkien explored various possibilities regarding the exact interpretation of the elements occurring in the name: it is either Azanul+bizar or Azan+ûl+bizar though the whole was to somehow express "Dimrill Dale"..
Tolkien stated that "the Common Speech form is an accurate translation: the valley of the dim (overshadowed) rills that ran down the mountainside". In another point it is given as "Vale of Dim Streams" with three elements.
The first interpretation says that azan "shadows, dimnesses", -ul genitive marker and bizar "streams, rills"; in this interpretation it means "rills of shadows", and the word "dale" is understood (the full name being duban Azanulbizar).
According to the second interpretation, bizar "dale" and ul "rill(s), streams".