Jamieson, Fausset and Brown on Isaiah 7:20

20. razor--The Assyrians are to be God's instrument of devastating Judea, just as a razor sweeps away all hair before it (Is 10:5; Eze 29:19, 20).

hired--alluding to Ahaz' hiring (2Ki 16:7, 8) Tiglath-pileser against Syria and Israel; namely,

by them beyond the river--namely, the Euphrates; the eastern boundary of Jewish geographical knowledge (Psa 72:8); the river which Abram crossed; the Nile also may be included (Is 7:18) [G. V. Smith]. Gesenius translates, "With a razor hired in the parts beyond the river."

head ... feet--the whole body, including the most honored parts. To cut the "beard" is the greatest indignity to an Easterner (Is 50:6; 2Sa 10:4, 5; Eze 5:1).