"Bend your breast" is still used very occasionally in modern English. It has much the same meaning as to "put your back into it". If the song had said that he "put his back into it and swam" would people be asking about what "it" was and whether he could put his back in without putting the rest in?

Similarly Beat your breast is not uncommon in modern English usage. Usually in phrases like "you don't need to beat your breast about it". In this case it means to express regret about something you think you may have done wrong (though not necessarily to do it honestly). I'm pretty sure this latter derives from the old latin mass, where people were expected to beat their breasts (chest) when saying "Mea Culpa" (I have sinned).

:-)