Something that sets you back. It keeps you from doing something and can cause great inconvenience to someones life. A setback may just knock you off your time schedule or it could be stopping you from doing something you drastically need to do. Setbacks are not good, most of the time...
"I really must get my sculpture finished today, but I don't have enough resources to finishes it. That means I'm gonna have to go out and buy some then I'll be off my time schedule. Dang!"
(^^^Not having enough resources to finish his/her sculpture was this persons setback)
Similar to "Fives" or calling "shotgun", "Quack Quack Seatback" is a common way to lay short-term claim to a seat you're about to vacate when with a larger group. Sometimes shortened to "Quacks."
Person A: "Hey, you stole my seat."
Person B: "It's my seat now."
Person C: "No, he called Quack Quack Seatback. Move it."