It is a derived from the idea of purchasing a soda at a restaurant and having unlimited ReFills to your hearts content.
*in terms of sex, a ReFill sometimes confused as a booty call, but instead is the close sister of a booty call*
A ReFill is having open sex access to ex-girlfriends or other girls who'm you've taken their virginity from thus allowing them to be attached to you indefinately.
Jeff: Who are you calling?
Dave: Ex-Gf. I need to get laid.
Jeff: Ahhhh, I too enjoy ReFills.