Someone that says they will do something then last minute backs out of it.
Matt promised he would come out tonight but he going to be sitting at home doing nothing, such a gup out!
A person who knows and uses all the chat rooms to troll and find friends, a gup out is also known as the most retarted person in your college class.
The fluff that stays and staines inside your jeans is often called a major gup out.
Seriously mate, your the biggest gup out I've ever met.