1: Something gross, low-class or unclean. Originally meaning neat in apperance, the word natty ironically became its an antonym for itself over time, thanks in large part to its adoption by Rastafarian slang. When the Bob Marley and the Wailers album Natty Dread was released, "natty" was used positively to describe the "dread" as hip or cool. When people realized that Rastafarians are stupid and don't shower and the "natty" dread is actually gross, the word changed meanings.
2: Natural Light or Natural Ice beers are commonly referred to as "Natty" instead of "Natural," reenforcing the new meaning of Natty (def 1).
3: A filterless, hand rolled cigarette that is very harsh on the lungs, smells terrible, and, if not rolled correctly, can be a chore to smoke.
1: "She's hot, but I heard she doesn't shave and has a real natty, hairy pussy."
2: "I wanted to get wasted at that party, but all they had was Natty Light, so I huffed gasoline instead."
3: "I hate smoking nattys, but rolling tobacco is just SO cheap!"