To emphasize something really good or bad, depending on how it is used in a sentence. Can have several meanings and can be used as a verb or a synonym.
That dude's shirt is not da business!
My husband gave me da business last night for coming home too late.
I love her hair. It is da business!
Boys who enjoy yaoi (a genre in Japan that contains sexual and/or romantic relations between two men); literally translates to "rotten boy"; corresponding female : fujoshi