The Collaborative International Dictionary of English v.0.48:

boff \boff\, boffo \bof"fo\n.
   1. (Theater) a hearty laugh.
      [PJC]

   2. (Theater) a joke or gag line that produces hearty
      laughter.
      [PJC]

   3. (Theater) something very successful, especially a
      box-office hit.
      [PJC]