Having read the other answers, I would say that this term means different things in different cultures. In my culture it is used often and usually refers to a state of mind, rather than a physical condition. "Fat" is often used to mean contented. "Sassy" suggests confidence and usually means overly confident or arrogantly confident.
Examples: "Considering my high score on the midterm exam for my English class, I am feeling fat and sassy about taking the final exam."
"With all of the attention the boys are giving her, Jane has gotten a fat and sassy attitude."