You would usually say it to chavs, gangsters, wiggas and anyone else who has NO sense of style, shows incorrect usage of the English language, and generally has incompetence written all over their faces.
'Iya Der Iz u Aite Blud?' - DAN
'Dan, you are definately a BWAB!
'Yo Momma iz so fat..dat she be.... -
'shut up you BWAB'