Bum, backside, the initial and now obsolete meaning is “the anus”. It is borrowed from the Greek word αφεδρών (anus) and is not quite a slang word but rather and elegant way of referring to the anatomy.
— Какого диаметра у вашей мамы, я извиняюсь, афедрон?
— What is the diameter of your mother’s, I apologize, backside?
(Грант Бородин, Прившин первым заметил неладное, 04/2005)