1
general::
Start working, become active, as in Stop dawdling; get busy, or We'd better get busy on this paper. [c. 1900] Also see GET A MOVE ON; GET GOING; GET ON THE STICK.
American Heritage Idioms
2
general::
Phrase(s): get busy
to start working; to work or appear to work harder or faster. • The boss is coming. You’d better get busy. • I’ve got to get busy and clean this house up.
McGrawhill's American Idioms And Phrasal Verbs