Dum Maaro Dum or Dum Maro Dum was a song composed by RD Burman and sung by Asha Bhosle, from the 1971 Hindi film Hare Rama Hare Krishna.
Dum Maro Dum literally means 'Puff, take a Puff' but if you want to be very clear, it would translate to something like Concentrate on taking the puff (and all your sorrow will be gone, as the next line goes).
Dum Maro Dum is also the title of an upcoming film by Rohan Sippy, starring Abhishek Bachchan.