This phenonmenon has been around since at least the 80's. It is defined as the elongation or widenty of a mother's butt when she wears jeans/pants that are too tight, too short, or too high in the waist. One does not have to be a mom to aquire mom's butt. One does also not need to be fat to aquire mom's butt. However, there is a vast cohort of Mom's Butt sporters at youth soccer games. A true mom's butt shows a panty-line that usually is uneven due to a "wedgie". Pockets are usually too small and too high on the pants, thus, creating the mom's butt shape.
Everyone can experience mom's butt. There is a make-your-own mom's butt method that you can do at home. First put on a pair of jeans. Second, pull them up as high as you can, almost to wear they cut you. Then bend over a little and look in a mirror from behind. MOM'S BUTT!