Used to describe something so completely epic, it is impossible to describe without saying "fuck." If the object can only be described by saying any form of "fuck," followed by another adjective, then said object is deemed "the fuckness."
Used to describe the sexual attractiveness of another person.
Fred: "Dude, did you see those tits? They were fucking amazing."
Jeff: "Oh my God yes, how could you not see them? They are the fuckness."
John: "Out of ten, how would you rate her fuckness?"
Steve: "Eh, probably a 9."
John: "So you'd bang?"
Steve: "Def. I'd fuck anything above a 7."