friends cup
when playing beer pong, there is always left overs(backwash) in the cups needed to play the next game. The friends cup is a place to pour all the backwash, or "friends".
person 1: is this my beer?!
person 2: DONT DRINK THAT!! ITS THE FRIENDS CUP!
person 2: DONT DRINK THAT!! ITS THE FRIENDS CUP!
plastic cup friend
A person you meet at an event or bar who is cool to hang out with at the time but you never talk to or hang out with again. A throw away friend.
He was really cool at the bar last night, but he's definitely a plastic cup friend.