Ye shall conceive chaff - An address of God to the Assyrians. The figure is one that denotes that their counsels would be in vain. Chaff and stubble are used in the Scriptures, in contrast with grain, to denote anything which is not solid, nutritious, or substantial; then anything which is frivolous, useless, vain. A similar image occurs in Isaiah 26:18 (see the note on that place; compare Isaiah 59:4).
Your breath as fire shall devour you - The word ‘breath’ here (רוח rûach, spirit) is evidently used in the sense of the Θυμός thumos, and denotes anger, as in Isaiah 30:28. It refers to the haughty and arrogant spirit of Sennacherib; the enraged and excited mind intent on victory and plunder. The sense is, that his mind, so intent on conquest - so proud, excited, and angry, would be the means of his own destruction. Lowth proposes to read ‘my spirit,’ but for this change there is no authority from manuscripts (see the notes at Isaiah 1:31).