CUNT is originally a variation of COUNTRY (girl). 500 years ago, when a young country woman went into the city, she often had to use her sexual skills to survive. This led to the degrogatory use of the abbreviated CUNT to refer to such a women and her vagina. All future uses are derived from this original definition.
Read Shakespere, where he refers to "COUNTRY MANNERS" as a sexual joke about a CUNT.