1 general:: Be in earnest. For example, He really means business with this deadline. This idiom uses business in the sense of "a serious endeavor." [Mid-1800s]
American Heritage Idioms
2 general:: Phrase(s): mean business to be very, very serious. • Stop laughing! I mean business. • I could tell from the look on her face that she meant business.
McGrawhill's American Idioms And Phrasal Verbs