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knowyourphrase.com - The Meanings and Origins For Several Common Idioms and Phrases
Phrases and Idioms
This is not the case for every phrase, but there will certainly be times where the origin of a phrase is not clear. In such cases, what you'll usually see are theories or guesses that take a gander as to where the phrase may have come from, but these may not be accurate.

Additionally, the quotes that I... quote, are typically the oldest that I can find. These come from old books, poems, or newspapers, etc. Keep in mind, though, that there could be an older recording that I possibly overlooked.
One should not form an opinion on someone or something based purely on what is seen on the surface, because after taking a deeper look, the person or thing may be very different than what was expected.
If someone is looking for a book to buy and read, the first thing that will probably grab their attention is the cover of the book. Based solely on the cover, a person may decide whether a book is or is not for them. As a result, they may overlook a book simply because the cover appears plain or uninteresting to them. However, if the person would have opened up the book and look at what's inside instead of overlooking it, they may have found it to be pretty interesting after all.

This expression is also applied to people. How so? Well, people are often judged solely based upon their outward appearance. However, if one were to get to know the person and see what's on the inside, 'opening' the person up, so to speak, then that one may be pleasantly suprised to find that the person is very different to how they imagined. Hence, this expression is commonly used as a warning that a person should not judge people or things simply by what they see on the outside.

The phrase goes back to at least the mid-19th century, as seen in the newspaper Piqua Democrat, June 1867:

"Don't judge a book by its cover, see a man by his cloth, as there is often a good deal of solid worth
and superior skill underneath a [???] jacket and yaller pants."

The print in the newspaper I was looking at was really small and hard to read, but even so, I tried quoting it as accurately as I could. Regardless, there was enough clarity to make out the phrase for sure.
* Pineapple never appealed to Jake due to its hard and spiky exterior, but a friend of his cut a slice for him to eat the other day, and Jake quickly learned that you shouldn't judge a book by its cover.
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