The Hotness rating is a rating out of 10 that you give to a person of the opposite sex. The hotter the person, the higher the rating. Therefore, Scarlett Johansson is regarded as a 10/10, and Rosie O'Donnell scores a lowly 1/10.
Dude 1: Hey, i scored with this chick last night!!
Dude 2: Was she a hottie?
Dude 1: I'd give her a hotness rating of 7/10.