No, that's not an idiom.
Student or Learner
At the magazine called "The Economist" I've read the following sentence:
" And, with unheard-of daring, he sniped at political structures as well as pseudo-science."
What's the meaning of "unheard of daring" ?
Is "unheard of daring" an american idiom ?
It means unprecedented boldness/courage.
Note that the hyphen in "unheard-of" is vital. It makes "unheard-of" an adjective.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.
You're a newbie, so you don't know. All you have to do to thank somebody is click Thanks. (You may thank me now.)