Its a type of party in which everyone must come with a date and a 750 ML bottle of hard alcohol (fifth). However, you must stay zip tied to your date until the two of you finish the entire bottle together. This usually leads to a shit show and everyone gets hammered. Its also convenient because the people throwing the party don't need to buy alcohol other than for themselves. Invented at Santa Clara University.
Joe: Hey are you going to date and a fifth with Katie?
Frank: Hell no, she barely drinks. I'm not trying to black out.