I shook my lap, i.e. the lap or extreme parts of my garment, which I first folded together, and then shook it, and scattered it asunder. This was a form of swearing then in use. From his labour, i.e. from enjoying what he hath got by his labour.
Related Commentaries of Nehemiah 5:13
Nehemiah 5:13
13 Also I shook my lap, and said, So God shake out every man from his house, and from his labour, that performeth not this promise, even thus be he shaken out, and emptied.b And all the congregation said, Amen, and praised the LORD. And the people did according to this promise.