The LORD is slow to anger, and great in power, and will not at all acquit the wicked: the LORD hath his way in the whirlwind and in the storm, and the clouds are the dust of his feet.
The Lord is slow to anger, and great in power - i:e., He is slow indeed to anger, but great in power, so as to be able in a moment, if He pleases, to destroy the wicked. His long-suffering is not from want of power to punish (Exodus 34:6-7, "The Lord, the Lord God, merciful and gracious, long-suffering ... forgiving iniquity and transgression and sin, and that will by no means clear the guilty." To this last clause Nahum's next clause alludes).
And will not at all acquit the wicked - literally, will not acquitting acquit, or treat as innocent.
The Lord hath his way in the whirlwind. From this to Nahum 1:5 inclusive is a description of His power exhibited in the phenomena of nature, especially when He is wroth. His vengeance shall sweep away the Assyrian foe like a whirlwind (Proverbs 10:25).
And the clouds are the dust of his feet - large as they are, He treads on them, as a man would on the small dust; He is Lord of the clouds, and uses them as He pleases.