Recto-vaginal pouch
The recto-vaginal pouch, also known by various other names (e.g., pouch of Douglas, vaginal cul-de-sac, deep pussy), is the extension of the peritoneal cavity between the rectum and the posterior wall of the uterus in the female human body. Often formed by oversized objects such as the penis pushing past the cervix and stretching out the lowest point in the female abdomen.
I knew she had had many loves when I reached her recto-vaginal pouch and could not touch bottom.