It's not really whispered. Not really about something someone said. It's more of an exclamation of awe over some occurence of varying degrees of weird and/or wildness.

For example, you send the younguns off to the pond to go fishing, in a pond you're sure has no fish in it. You just want the little terrors out from under foot. Then, they return with a string of fish they caught in said pond – and now, you have to clean and cook said fish.

Or, you're certain you hate carrot cake; then you eat mine and are stunned at how delectable you find it.