1) While doing a female doggy style, the male withdraws his penis from the female’s vagina prior to ejaculation during intercourse (coitus interruptus), at which this time, the semen ejaculated onto the female’s back is then being collected in the dimples of the female’s lower back.
2) While doing a girl from behind, the guy pulls out and nuts on the girl’s back. The jizz is then collected in the dimples of the girl’s lower back.
3) Indentations on a girl's lower back that can be used as thumb grips during doggy style prior to becoming filled with semen.
Synonyms: 1) Dimples of Venus; 2) Sacral dimples; 3) or simply, back dimples.
"I was at the gym (or beach) yesterday and I saw a girl with cum reservoirs on her back!"
"Ughhh... yeah, do me doggy style baby, and before you cum, I want you to pull out and fill up my cum reservoirs with your hot load!"
"Hey, wasn’t that scene in "Kick-Ass" awesome when Kick-Ass was rubbing lotion his friend’s back, and the camera panned down for a perfect close-up of her cum reservoirs?!?"