Jamieson, Fausset and Brown on Genesis 15:3

3. Eliezer of Damascus ... one born in my house is mine heir--According to the usage of nomadic tribes, his chief confidential servant, would be heir to his possessions and honors. But this man could have become his son only by adoption; and how sadly would that have come short of the parental hopes he had been encouraged to entertain! His language betrayed a latent spirit of fretfulness or perhaps a temporary failure in the very virtue for which he is so renowned--and absolute submission to God's time, as well as way, of accomplishing His promise.