This is a Navajo word for sleepiness, or drowsiness.
This form of bił, in contrast to the similar sounding shił, nił, bił, and nihił (which mean “with me”, “with you”, and so on), can be considered a noun that does not change with the point of view. It can refer to anyone.
Use this word the same way you use “dichin” (hunger) and “baa ahééh” (about it thankful…), with the verb “nisin” (I want it; it is on my mind; etc.). You should begin to notice how useful it is to know the “nisin” verb forms.
Da’ bił nínízin? Are you are sleepy?
Bił nisin. I am sleepy.