Chaff is the husks of the grain that would be swiftly blown away from a threshing floor by even a slight breeze. The imagery used in this phrase in Zephaniah 2:3 at first sight seems to trivialize the day of the Lord. But the image underlines the shortness of time available until the day of the Lord. As chaff is easily blown away by a gust of wind, so the time intervening between the present moment and the descent of the fierce wrath of Yahweh passes away in an instant. No time remains to dillydally before that Day.
1
2 before the decree takes effect before the day passes away like chaff before there comes upon you the burning anger of the LORD, before there comes upon you the day of the anger of the LORD.