1 Good. Originally from the terminology of the poorest black Americans, either as simple irony or based on the assumption that what is bad in the eyes of the white establishment is good for them, this usage spread via jazz musicians in the 1950s to teenagers in the 1970s. It is still primarily a black term, although it is occasionally used, rather self-consciously, by white teenagers in the USA and, under the influence of rap and hip hop, in Britain since the early 1980s. This use of bad is normally distinguished fromits opposite, literal meaning by a long drawn-out pronunciation. The superlative form is 'baddest'.

'In hip hop slang "that's bad" can mean "that's good", depending on the tone of voice.' (Evening Standard, 11 November 1987)

2. As noun: (American)

a fault, mistake. A key item of black street slang that was adopted by white adolescents in the 1990s, usually in the form 'my bad!', an acknowledgment of guilt or blame.