In a living arrangement that contains 3 lease holders in one household, it is predetermined that power is distributed equally into 33% per person. 66/33 is when two of the roommates start to date and their decision making process becomes parallel, making the power distribution 66% to 33%. This makes the third wheel in the household unable to win any argument against the couple. A 66/33 often results in the third party moving out, or being kicked out of said living arrangement due to being outnumbered.
Person 1: "Yeah, I just moved out of my apartment the other day."
Person 2: "Why is that?"
Person 1: "66/33, she was sleeping with my other roommate and they wanted it to be a 'couples' household."