not what it's cracked up to be

Example: “The famous restaurant wasn't what it's cracked up to be.”

Origin: "Crack" once meant to boast or talk something up, a sense attested from the 1400s and long since vanished from ordinary use except inside a handful of fossils. It survives in the adjective — crack troops, a crack shot, a crack regiment — where it still means first-rate, and it is the whole content of this phrase: what a thing is cracked up to be is what it has been boasted of being. The idiom therefore does not say the thing is bad, only that the claims made for it outran it, which is why it is nearly always used of something with a reputation attached. The construction is attested in English from the 1800s, by which point the underlying verb was already becoming obscure.