According to the Cambridge Advanced Learner’s Dictionary & Thesaurus definition:
Beat your breast/chest
To show how sad or guilty you feel in an obvious or public way
I understand what is meant by beating one's breast, but why is this the preferred way to express sadness and/or guilt? Apes beat their breasts also, but not with the same aim.
Isaiah 32:12-13 (ESV)
Beat your breasts for the pleasant fields, for the fruitful vine, for the soil of my people growing up in thorns and briers, yes, for all the joyous houses in the exultant city.
Luke 18:13 (ESV)
But the tax collector, standing far off, would not even lift up his eyes to heaven, but beat his breast, saying, ‘God, be merciful to me, a sinner!’
Luke 23:48 (ESV)
And all the crowds that had assembled for this spectacle, when they saw what had taken place, returned home beating their breasts.
(n.d.). Retrieved July 29, 2016, from http://dictionary.cambridge.org/dictionary/english/beat-your-breast-chest