After having sex missionary style, ask your girlfriend to turn over so that you can begin doggy style. While she's turning over, secretly replace your piece with a dildo of the same size and stature (make sure she can't tell the difference.) Once you start doing it with the dildo, after a while, fall off the bed leaving the dildo inside of her making her think that your junk fell off.
"I was hittin' my girl from behind when I decided to pull a dis-member on her... She almost had a heart attack when she thought my shit fell off."
When you remember but it's a really painful memory so you want to distract your discomfort with some ugly transfiguration of a word that just happens to be "I remember". A contraction of I dis-remember.
Oh yeah, those women who are 26 but look like 45 because of extreme crack abuse. Now I dismember.