Usually refers to men. Especially men who don't pay much attention to their looks, in terms of grooming, hair and clothes.
However when they put some attention to it they look good.
Best example is Shaggy from
Scooby-Doo. He always looks
frumpy, stained, unshaven, slump, but you always
get the feeling that when he needs to escort Daphne
Blake to her gf wedding Shaggy after a shave and a nice getup will look good.
Girl 1: Are you
crazy you want me to go out with Jonah Hill,
yuck
Girl 2: Sweety, he cleans up good and will make you look great on that red
carpet