Dating someone who is of lower caliber than you. Often done to hurt your ex.
Lowering their status as you lower yours.
After they broke up, Bill started dating-down with full figured women, when asked by friends, he stated you know I like big women. his ex who was size 8 went on a crash diet.
dating someone that is on a lower caliber as yourself.
Or one who is not excelled as far a you have in any aspect
example: The chairman of a company dates a mailroom clerk
When we met I knew I was dating-down but he made my heart melt.