Is this usage of revenir an idiom?

Question

Elle n'en revenait pas! I thought revenir is to come back so this phrasing confused me till I used Google translate to realize it meant, She couldn't believe it.

Accepted Answer

Yes, ‘n’en pas revenir ‘ is a fixed expression to indicate extreme surprise.
 https://www.wordreference.com/fren/ne%20pas%20en%20revenir
 https://www.larousse.fr/dictionnaires/francais/revenir/69087#locution

Answer

Think of the English phrase "to have no comeback to something."

Answer

Following on from Maarten’s link to WordReference. I find this free dictionary app great for listing phrases. If the literal translation isn’t making sense, it may well be a phrase, in which case WR will usually have the explanation (in my experience).