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Failing to meet expectations; not being as good as people say.
Note: The origins for most phrases and sayings cannot be said with a certainty. What's provided are theories that may be plausible to how a phrase originated, but not necessarily so.

In addition, quotes that contain a particular phrase may be taken from old newspapers, poems, or books that were written centuries ago, but this by no means confirms that the phrase originates from said newspapers, poems, or books. In all likelihood, if an expression is being used in a newspaper, it's probably already a well known saying and is from an older time.
* Ashley gave me a book to read, saying it was her favorite book of all time, but after reading parts of it this morning, I would have to say it's not all it's cracked up to be.
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The word "craic" or "crack" is said to have derived from the Middle English word crak,
which means "loud conversation, bragging talk." In some parts of the world, the term
crack is used to mean "news" or "gossip," which led to the expression "What's the crack?"
That is similar to asking: "What's the news?" When understanding the word in this context,
I could possibly see the phrase originating from something as described below:

When it comes to news or gossip, it's not unheard of for a person to speak boastfully about
someone or something. For example, a friend may speak highly of a particular restaurant,
saying the food there tastes incredible. Due to your friend's positive review, you decide to
check the place out for yourself, only to be extremely disappointed. Hence, you might say the restaurant they boasted so much about, or their crack, failed to meet expectations. The place wasn't all it was bragged (or cracked) up to be.

Davy Crockett, a politician, used the phrase in the year 1835 when he commented on someone running for president:

"Martin Van Buren is not the man he is cracked up to be."

Reference: Wikpedia - Craic