"thump your chest"

What does "thump your chest" mean?
e.g. It may be smart to thump your chest and critize your friend.

Actually, it may be *hypocritical* to thump your chest while criticizing your friend. In the old days (pre-1965 or so), when Roman Catholic Mass was in L@tin, the congregation would say together "Mea cula, mea culpa, mea maxima culpa", while striking the chest three times. The phrase means roughly "I have sinned, I have sinned, I have sinned terribly".