Charles Ellicott Commentary Isaiah 7:15

"Butter and honey shall he eat, when he knoweth to refuse the evil, and choose the good." — Isaiah 7:15 (ASV)

Butter and honey he shall eat, that he may know ... — A better translation is, until he knows, or, when he knows .... By a strange inversion of the familiar associations of the phrase (Exodus 3:17; Deuteronomy 31:20), these words describe a time not of plenty, but of scarcity. The prophet probably spoke them with a certain touch of ironic paradox. (Compare to Isaiah 7:22.)

Fields and vineyards would be left uncultivated (Isaiah 5:9). Instead of bread, meat, wine, and oil, the people, fleeing from their cities and taking refuge in caves and mountains, would be left to the food of a nomadic tribe, such as, e.g., the Kenites (Judges 5:25; 1 Samuel 14:26; Matthew 3:4).

The “butter” of the Bible here, as in Judges 5:25, is the clotted milk which has always been a delicacy with Arabs.