Originated at a Taco Bell, whereupon the creator was deciding whether or not to buy a Caramel Apple Empanada. The creator had no idea what an empanada was, but caramel and apple made it sound 2/3 good. The Caramel Apple Empanada turned out to be delicious, and thus the 2/3 rule was born.
Friend of indecisive customer: "I don't know, dude, but it sounds 2/3 good. By 2/3 rule, you have to try it."