Literally meaning the ass hole and how a person has never had anal sex. Behind-a-hymen denotes a person male or female, being a virgin to anal sex. Therefore, not having their ass (behind) hymen broken.
Adam Carolla was the first person to coin this term on his morning radio show and has used it a number of times.
Josh: "I tried to have anal with Teresa the other night and she got super pissed at me."
Corey: "No way!! She still has her behind-a-hymen at 24? There's just no way!"