Jamieson, Fausset and Brown on Isaiah 7:3

3. Go forth--out of the city, to the place where Ahaz was superintending the works for defense and the cutting off of the water supply from the enemy, and securing it to the city. So Is 22:9; 2Ch 32:4.

Shearjashub--that is, A remnant shall return (Is 6:13). His very name (compare Is 7:14; Is 8:3) was a standing memorial to Ahaz and the Jews that the nation should not, notwithstanding the general calamity (Is 7:17-25; Is 8:6-8), be utterly destroyed (Is 10:21, 22).

conduit--an aqueduct from the pool or reservoir for the supply of the city. At the foot of Zion was Fount Siloah (Is 8:6; Ne 3:15; Joh 9:7), called also Gihon, on the west of Jerusalem (2Ch 32:30). Two pools were supplied from it, the Upper, or Old (Is 22:11), or King's (Ne 2:14), and the Lower (Is 22:9), which received the superfluous waters of the upper. The upper pool is still to be seen, about seven hundred yards from the Jaffa gate. The highway leading to the fullers' field, which was in a position near water for the purposes of washing, previous to drying and bleaching, the cloth, was probably alongside the aqueduct.