Like a baby bump, except your not large from being pregnant, but large from eating too much and being lazy and not taking care of yourself cause you are in a relationship and dont need to put an effort into keeping yourself looking sexy.
When you go from being a sexy girl that everyone wants to tap, to then having a boyfriend and then 3 months later youre putting on weight and have a belly, thats a Relationship bump!
Eg1: Oh look what happened to Sarah, after she started dating Steve shes gotten all fat! Look at that Relationship Bump man! What happened to that sexy thang, she used to look as fly as Jlo!!
Eg2: I really need to hit the gym and work off my relationship bump. This boyfriend of mine is got me too comfy and making me lazy!