Adam Clarke on Jeremiah 16:6

Nor cut themselves - A custom of the heathen forbidden to the Jews, Lev 19:28; Deu 14:1, and which appears now to have prevailed among them; because, having become idolaters, they conformed to all the customs of the heathen. They tore their hair, rent their garments, cut their hands, arms, and faces. These were not only signs of sorrow but were even supposed to give ease to the dead, and appease the angry deities. The Hindoos, on the death of a relation, express their grief by loud lamentations, and not unfrequently bruise themselves in an agony of grief with whatever they can lay hold on.