Charles Ellicott Commentary Genesis 27:39

"And Isaac his father answered and said unto him, Behold, of the fatness of the earth shall be thy dwelling, And of the dew of heaven from above." — Genesis 27:39 (ASV)

Isaac his father answered. —Unwillingly, and only after repeated entreaty and earnest expostulation, and even tears from Esau, Isaac brings himself to the effort to lessen in any way the painful consequences for his favorite son of his brother having robbed him of the blessing. Plainly, he felt that he had endeavored to do what was wrong and was afraid that he might still be found resisting God’s will.

Your dwelling will be the fatness. —Hebrew, your dwelling will be of the fat places of the earth (see note on Genesis 27:28). But most modern expositors consider that the preposition should not be translated “of,” but from; that is:

“Behold, your dwelling will be away from the fat places of the earth,
And away from the dew of heaven from above,
And by (Hebrew, upon—depending upon) your sword you will live,” etc.

By this rendering, the parts of the blessing agree together. Those who have fertile lands live by agriculture, but the inhabitants of sterile regions must look to more adventurous enterprises for a living.

So the Swiss, like the Greeks of old, long served as mercenaries in the armies of other states. Idumæa, though not destitute of fruitful tracts, and even famous for its orchards, was, as a whole, sterile and unproductive, and the people were restless and unsettled.

Moreover, Isaac had already given the corn-land and vineyards to Jacob (Genesis 27:37), and he had no power to give them as a second gift.

It is not an answer to this to say that because the same preposition is used in Genesis 27:28, it cannot have a contrary sense in the two blessings. There, it follows a verb of giving and necessarily has a partitive signification. Here, there is nothing that absolutely settles its meaning, and we are left to the general sense.

Possibly, Isaac may have purposely used an ambiguous word; but the meaning as a whole is clear. Esau was to inhabit a land which by its barrenness would force him to a life of adventure, military service, and freebooting.