Albert Barnes Commentary Proverbs 26:18-19

"As a madman who casteth firebrands, Arrows, and death, So is the man that deceiveth his neighbor, And saith, Am not I in sport?" — Proverbs 26:18-19 (ASV)

The teacher cuts off the plea which people make when they have hurt their neighbor by lies, that they “did not mean mischief,” that they were “only in fun.” Such jesting is like that of the madman flinging firebrands or arrows.