A Thick Bandit is often reference to the large female in the herd. She will tend to be more dominant, and more likely to perform exotic mating rituals with the male, and often times and added female.
"Did you see that Thick Bandit last night? Yeah, I took one for the team, took her home...and she destroyed me!"
"Mike was wing manning for a buddy, and ended up with the Thick Bandit...He'll never be the same."
"Had a thick bandit last night, and she let me go for the dog in a bathtub."