Adam Clarke's Commentary on the Bible

Verse 9 (Ecclesiastes 7:9)

Anger resteth in the bosom of fools - A wise man, off his guard, may feel it for a moment: but in him it cannot rest: it is a fire which he immediately casts out of his breast. But the fool - the man who is under the dominion of his own tempers, harbors and fosters it, till it takes the form of malice, and then excites him to seek full revenge on those whom he deems enemies. Hence that class of dangerous and empty fools called duellists.

- Adam Clarke's Commentary on the Bible