1slang A successful pimp or criminal, especially one who acts or dresses in a way regarded as (stereotypically) characteristic of such a person; (hence, more generally) any person who acts or dresses in this way.
In early use as the nickname of the protagonist of an anonymous African-American narrative poem.
2slang A supremely successful, respected, or influential person; specifically a man who is extremely attractive to or sexually successful with women, a playboy.
3slang, figurative A person who or thing which is considered a supreme or model example of something. Usually followed by of.
1960s. Probably from mack + daddy, in some forms apparently after Mc-.