It is used when calling someone a skank but with more emphasis so they sound better. Skank was used when people didnt swear now they do they still use the old words with swearing.
a gurl is sat on her own that doesnt have the best style or something like that another gurl comes past with a group of friends and they are being bitchy and 1 of them says "what a skank bitch" then goes back to the conversation she had before.
It is also used in different terms e.g skank whore, skank hoe e.t.c. then those terms are used when in a bitchin fight.