Isaiah 27:12

the Lord shall beat off … Egypt Rather: Jehovah shall thresh out from the corn-ears of the River (the Euphrates) unto (those of) the brook of Egypt, i.e. all that grows between those limits. The term "beat out" is applied both to the beating of olives from the tree (Deuteronomy 24:20) and to the beating out of grain with a staff a more careful process than the ordinary methods (Isaiah 28:27; Judges 6:11). The latter analogy gives the best sense here. The "brook of Egypt" is the Wadi el Arish, the south-western frontier of Palestine, this and the Euphrates being the extreme boundaries of the ideal territory of Israel (Genesis 15:18, &c.). The meaning is that within this territory Jehovah will carefully separate the corn from the chaff and straw, the true Israelites from heathens and apostates, Isaiah 27:13 then describes, under another figure, the ingathering of those who were exiled beyond these limits.