Like a drunken blind-date, it is the act of banging somone at a party while you are both legless, and having no memeory of who the actul person was the morning after. You do, however, have evidence (condom wrappers, lipstick etc). Its never happened to me, personally.
You know how was off his face from crownies at Simmo's rave last night? Well, apparently he woke up after a mystery meet with only two throbbing heads and a ear-ring to go on.