Jamieson, Fausset and Brown on Isaiah 21:10

10. my threshing--that is, my people (the Jews) trodden down by Babylon.

corn of my floor--Hebrew, "my son of the floor," that is, my people, treated as corn laid on the floor for threshing; implying, too, that by affliction, a remnant (grain) would be separated from the ungodly (chaff) [Maurer]. Horsley translates, "O thou object of my unremitting prophetic pains." See Is 28:27, 28. Some, from Jr 51:33, make Babylon the object of the threshing; but Isaiah is plainly addressing his countrymen, as the next words show, not the Babylonians.

Is 21:11, 12. A Prophecy to the Idumeans Who Taunted the Afflicted Jews in the Babylonish Captivity.

One out of Seir asks, What of the night? Is there a hope of the dawn of deliverance? Isaiah replies, The morning is beginning to dawn (to us); but night is also coming (to you). Compare Psa 137:7. The Hebrew captives would be delivered, and taunting Edom punished. If the Idumean wish to ask again, he may do so; if he wishes an answer of peace for his country, then let him "return (repent), come" [Barnes].