1. A hamburger consisting of three (or four, sometimes five) beef patties, each topped with a cheese of your choice. The meat is slapped on a huge bun, and can be topped with other things (onions, mustard, ketchup, pickles, etc.) May also be called phatburger
in some areas. Not usually found in chain restaurants, but a staple of greasy spoons on the corner. The burger called a fatburger because of its size, and because eating a lot of them will probably make you fat.
2. Depending on where you get it from, it can be the best burger you'll ever have....when it's cooked properly.
I'm soooo hungry. Time to get a fatburger.
Gimme a fatburger with everything on it....extra onions. Make it five slabs of meat, please.