Cadaverous means looking corpse-like, from being sick or too skinny, like an aging rock star or a Halloween ghoul.
The adjective cadaverous literally means "like a cadaver." It's from the Latin word, well, cadaver (dead body), that comes from another Latin word cadere meaning "to fall." That's some fall! You don't have to be dead to look cadaverous, though, if your cheeks are sunken and your ribs poke through your torn tee-shirt, you look cadaverous and it's not a compliment (unless you're a Goth). On Halloween, you might go for a cadaverous look with green-tinted makeup and black circles under your eyes.