What does Genesis 27:40 mean?

In the previous verse, Isaac began a response to Esau's request for a blessing. Only, it doesn't sound like a blessing. Isaac's original intent was to bless his older son, Esau, but the plan was overheard by the boy's mother, Rebekah (Genesis 27:1–6). She and Jacob pulled off a scam to impersonate Esau, fooling the elderly and blind Isaac into blessing the wrong son (Genesis 27:5–7). Both mother and father are motivated by blatant favoritism (Genesis 25:28). In Isaac's case, this meant that his blessing bestowed almost total dominance and success of one son over the other (Genesis 27:27–29). While Esau hopes to gain something—anything—from his father, Isaac has left nothing positive to grant now that he has been fooled.

Instead, Isaac has described the life of a nomad wandering the desolate places of the earth away from the dew of heaven and the fat of the land. Isaac continues here by predicting that Esau will live by the sword and serve his brother. One day, though, he will grow restless and break free of Jacob's rule.

The offspring of Jacob and Esau will clearly fulfill these prophesies. Esau's descendants became the Edomites, a violent and warring people who did in fact serve Israel under David's rule only to break free and take their revenge (2 Kings 8:20–22). The eventual doom of these people is predicted in the brief book of Obadiah.