When you take advantage of a friend of either sex at a party who's drunk and willing.
Last night, my best friend and I ended up in a furff after about 5 drinks.
At that party last week I furffed Sally. She'll never talk to me again.
It can also mean the person that furffed. I've been hearing this more and more. Some say it stands for "F'd Up a Really Fantastic Friendship".
As in, "Gee Harold why you gotta be such a furff? You've known her since gradescool!"