허리 means "the part of a body between rib and buttocks" which is waist and the lower part of back. 등 means "the opposite side of chest and abdomen" which is the upper part of back. 등허리 means "the lower part of back" "I have a backache." can be mean 등이 아파요. (upper back pain) or (등)허리가 아파요. (lower back pain)