A hottie is a person who looks very attractive in a sexy way. It is informal slang: the kind of word friends throw around after someone walks past, not a careful description and not a kind word to say to someone's face unless you are sure they will like it. People use it for women and for men now, though you still hear it a little more about women. It comes from hot in the “good-looking / sexy” sense, plus a small familiar ending. Though actually, in some British homes an older person may call a hot-water bottle a hottie too — that is a completely different thing. There is also old slang for a stolen car, perhaps from hot-wired, but that is not what people mean when they point at a person.