What does Isaiah 7:21 mean?

The prophet Isaiah is describing the judgment the Lord will bring on Judah through the invading armies of Assyria. He has said that the armies will arrive in Judah as infesting insects (Isaiah 7:18). The Lord will use the king of Assyria as a razor to humiliate His people (Isaiah 7:20). In this period and culture, to be shaved against your will was a grave dishonor and the original audience would deeply understand the point the Lord is making.

Now Isaiah describes something that sounds positive but reveals just how many people will be dead or exiled. He pictures a man who has figured out how to keep one young cow and two sheep, or goats, alive. This implies the man can come up with enough feed for three animals. The following verse (Isaiah 7:22) will say that such a man will have an excess of milk to drink. This will happen either because those animals will have no young to feed or because the population has been diminished to such a degree that those who remain will have all the dairy they can use.