After a night of sex or other rowdy tomfoolery, a person would ask the person who was enjoying the sex, whether or not they where succesful, commonly used when a virginity is taken.
Whent he prefix Un is used, it shows a "Unsucsexful" night.
Dave: I heard you lost your V last night
Dave: Where you sucsexful?
Guy: TBH NO! i was unsucsexful.