The story
For the meaning development compare Proto-Slavic *plakati ultimately from *pleh₂k-, *pleh₂g- (“to beat”) (whence also Latin plangō (whence plangent, plain)).
Etymology from Wiktionary, CC BY-SA 4.0.
beat one's breast
Today it means: To ostentatiously display remorse, sorrow, or upset. Full definition →