Jamieson, Fausset and Brown on Isaiah 17:5

5. harvestman, &c.--The inhabitants and wealth of Israel shall be swept away, and but few left behind just as the husbandman gathers the corn and the fruit, and leaves only a few gleaning ears and grapes (2Ki 18:9-11).

with his arm--He collects the standing grain with one arm, so that he can cut it with the sickle in the other hand.

Rephaim--a fertile plain at the southwest of Jerusalem toward Beth-lehem and the country of the Philistines (2Sa 5:18-22).