tilly

This is the term you would use as a substitute for the word 'vagina' if your mom didn't want you to know the real word. You probably thought that 'tilly' was the actual word for vagina until you were in the fifth grade and went over to a friend's house, discovering their dog was named Tilly and in turn being horrified.

"My mom told me that the vagina was actually called a tilly and now I can't call it by any other term."
"My camp counselor calls her vagina a tilly."
"You are hereby not allowed to tell a boy the true meaning of tilly until you are married."