One should not form opinions about someone or something based solely on external appearance.
While the sentiment is ancient, this specific metaphor emerged in the mid-19th century as book publishing evolved and decorative covers became common. The phrase gained popularity in American English around the 1920s-1940s, perfectly timing with the rise of mass-market paperbacks with often misleading or sensational cover art.
This phrase became especially relevant during the paperback revolution when publishers discovered that lurid, often misleading covers could boost sales dramatically. Ironically, the publishing industry both created the problem this phrase warns against and proved its wisdom - many literary classics were repackaged with completely inappropriate cover art!
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