This is one mode of expressing great grief among the Orientals, especially in mourning for the dead; and its insertion in the parable is very expressive of the deep sorrow of the penitent publican. His grief on account of his sins was like the grief of those who mourned for their dead.
Morier gives an interesting account of the ceremonies observed annually in Persia in commemoration of the death of Hossein, the grandson of Mohammed, who was slain. One part of the ceremonies consists in beating the breast as a token of grief. Morier says: “In front of the palace a circle of the king’s own tribe were standing barefooted, and beating their breasts in cadence to the chanting of one who stood in the center, and with whom they now and then joined their voices in chorus. Smiting the breast is a universal act throughout the mourning; and the breast is made bare for that purpose by unbuttoning the top of the shirt” (Second Journey, pp. 178-179).