The bottom/ボトム(Botomu)/底部(Dǐbù)

The bottom is ,the part of your body that you sit on ,a large padded area at the base of the back.
 It has many alternative names (bum, ass, fanny, etc.), indicating its significance.

Lower Body Cross Training Exercises : How to Do an Air Squat Exercise


Lower Body Exercises : How to Do Squats Without Hurting Your Knees & Lower Back