Green’s Dictionary of Slang

(US) an excl. indicating suddenness, abruptness, agreement.

1942		W. Winchell On Broadway 1 July [synd. col.] A guy gets himself down as a loyal American and — boffo — Hitler quotes him on the short wave.
1945		F. Fay How To Be Poor 132: But the kid — boffo! — he’s a goner.
2001		S. King Dreamcatcher 223: ‘How are we going to come out of this, Pearly?’ ‘Clean?’ ‘Boffo!’.