A nasty is someone who does not bathe, brush their teeth, or even attempt any sort of hygienic activity.
They are typically very loud and obnoxious, and cannot comprehend normal social etiquette. They like to loom in the distance and listen in on normal people's conversations, walk too close to you, and generally act creepy.
Nasties tend to group together, and it is not uncommon for them to become a clique, where they will proceed to inter-date and fester in their own little worlds.
Everyone knows a nasty. No one can escape their greasy clutches. The best you can do is avoid eye contact and sit as far away from them as physically possible. Becoming friends with them will only make you miserable and a social outcast.
Student: Oh... uh, hi.
Nasty: We used to ride the same bus. Remember that hat you let me borrow?
Nasty: I still have it. It still smells like you, too, lol.
Student 1: Oh my fucking god, that nasty is creepy as HELL.
Student 2: Fucking nasties.
Prices shown in USD.