Wordnet 3.0
ADJECTIVE (1)
1. 
 resoundingly successful and popular; 
- Example: "for years he was a boffo box office certainty"
The Collaborative International Dictionary of English v.0.48:
boffo \bof"fo\ a.
   highly successful; superbly well done. [slang]
   [PJC]
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]
WordNet (r) 3.0 (2006):
boffo
    adj 1: resoundingly successful and popular; "for years he was a
           boffo box office certainty"