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.
AdamCarolla 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 analwith 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!"