When a group of friends has all hooked up with the people within that group of friends.
Lacey: "Well, I hooked up with John last weekend and Chad the weekend before that."
Dana: "Oh John is a great kisser, I hooked up with him all last summer. But I think I like Chad better in bed."
Lacey: "Hm. Maybe I should ditch them and give Blake a try."
Dana: "Oh he's good too!"
Lacey: "Wow, this is total frincest."