Green’s Dictionary of Slang

1. a big laugh, a very funny joke.

1934		R. Nichols Fisbo Bk II XVII 58: I don’t like his map, his boffos or his frail.
1950		Sat. Rev. Lit. (US) 4 Nov. 27: The boffo, he said, is the laugh that kills, and he lamented the loss of the boffo in talking comedy. Tittering, yes; an occasional yowl, a rare belly laugh. But the art of the boffo seemed gone forever .
1969		in H.S. Thompson Great Shark Hunt (1980) 209: Nixon’s reaction to this boffo was not reported in the press.
1985		R. Frede Nurses 338: The biggest current jokes, I mean real boffos, are [...] the house officers who have wilfully gone out on strike.
1992		New Republic 6 July 26: A performer...out only for boffos wouldn’t have mentioned the suicide [HDAS].

2. (Aus.) a joker, a clown.

1987

Sydney Morn. Herald 9 Nov. 12/2: [A] band of boffos known as the Protectors, named after the furniture polish, or, perhaps, the athlete’s codpiece.