A stereotype is used to catergorize a group of people. People don't understand that type of person, so they put them into classifications, thinking that everyone who is that needs to be like that, or anyone who acts like their classifications is one.
Stereotype for Goths are black clothes, black makeup, depressed, hated by society.
Stereotype for Punks are mohawks, spikes, chains, menace to society, always getting in trouble.
"THUNDER THIGHS" are a fat girls thighs that are so humoungous, big & fat that they clap together when they walk. This produces a loud noise which goes like "SLAP, SLAP, SLAP" , hence the "thunder" part of the word.