What does it mean to "beat one's breast"?

"To beat one's breast" (see definition 18 at the linked site) literally refers to striking your fist against your chest as an expression of sorrow, anger, or woe. Hardly anyone actually beats their breast any more, but the expression survives as a poetic expression that describes any act of public bewailing.

"Beating one's breast" is an expression of grief, disappointment, or woe. It is not a common action, but as an expression, it is usually figurative. Dictionary.com says it is an idiom which means "to display one's grief, remorse, etc., in a loud and demonstrative manner".

The idiom derives from older times, when people would actually do it.

The meaning reported from the NOAD for beat one's breast is "make an exaggerated show of sorrow, despair, or regret."

To beat one's breast was a sign of sadness, or more commonly, penitence and remorse. It's basically humiliating yourself by hitting yourself.

One of the earliest examples of a man doing it, would be in Jesus' parable of the two men who went to pray in the Temple, and one of them was remorseful and penitent, and was beating his breast as he prayed:

And the publican, standing afar off, would not lift up so much as his eyes unto heaven, but smote upon his breast, saying, God be merciful to me a sinner.