Charles Ellicott Commentary Genesis 27:40

"And by thy sword shalt thou live, and thou shalt serve thy brother. And it shall come to pass, when thou shalt break loose, That thou shalt shake his yoke from off thy neck." — Genesis 27:40 (ASV)

When thou shalt have the dominion.—This rendering of a rare and difficult Hebrew word is scarcely more than a guess made by two or three ancient Jewish commentators. Its real meaning here, and in Jeremiah 2:31, Hosea 11:12, is to toss the yoke—to be restless and unquiet.

The prophecy of Edom’s subjection to his brother was literally fulfilled, as Idumæa was for ages a mere dependency upon Judah. However, first in the days of Joram, and then of Ahaz, it revolted and recovered its freedom. It was again conquered by Hyrcanus, the nephew of Judas Maccabaeus; nor was its subject condition altered by the fact that the dynasty of the Herods was of Edomite extraction. In troubled times, then, it broke the yoke from its neck; but generally Edom served his brother.