Most muscular or strongest; superlative form of beefy, describing the greatest amount of muscle or beef.
From 'beef' + superlative suffix '-iest'; standard English formation of the highest degree of the beefy adjective.
Superlatives like 'beefiest' show how quickly new slang becomes grammatically integrated—people instinctively knew to add '-iest' to 'beefy' even though it's relatively recent vocabulary.
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