Slang from the 1920s. Possibly a corruption of the word business. Another possibility is that it is derived from the expression, "be all end all" a.k.a. "B's and E's" which when said would sound like, "Bees-an-ees". Simply defined as the best, latest, and greatest. (Synonymous with other 1920s slang: The cat's pajamas and the cat's meow. A more current version would be The shit.).
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