A big stack bully is the guy or girl having the biggest stack in a poker game with the power to outbet the smaller players. They can easily scare you into folding your good hand when you cant match their bets.
Bill Gates: "Yo, imma 'bout to buy a billi-sack hoe"
Bill Gates Whore Wife: "No Bill, we have to cure AIDS or some shit!"
Bill Gates: "Fuck that, I'm getting high."