Sensational, highly successful, or impressive—often used in entertainment and theater to describe something that's a huge hit.
From theatrical slang, possibly from 'boff' (to hit) suggesting a knock-out success, or imitative of a striking sound. Popular in American entertainment industry mid-20th century.
Boffo comes straight from theater lingo where critics would literally use it to describe shows that were absolute hits—it's the kind of old entertainment slang that makes you realize industry people always invent their own languages.
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