Short for Five Guys Burgers and Fries, a semi-fast food chain that originated in the DC area but now is across the whole Eastern United States (there is one near where I live in Indianapolis). They have very good, albeit very greasy, burgers and fries. They give very generous portions: a large order of Fries is easily enough for four average-appetite men to finish, and a small usually takes a few hours to eat. All in all, a very good and reasonably prices resturant.
I wish I could have Five Guys for lunch every day. I don't mind the heart attack at 35.