This refers to when a girl has such a large ass that she can "fill up" her Nike shorts. In other words the shorts become more like tight volleyball shorts rather than looking baggy
Do u see that girls butt? I bet she could be "filling out nikes"
By Belicia
Knocking Her Fillings Out
To make love to a woman in a hard energetic mannor. So much so that it may loosen the fillings in her teeth.