Good grief!
Definition 1 of 1
Pro Tip 1/3Mild Exclamation
Used for mild annoyance or surprise, not for serious grief or anger. 
Good grief, I forgot my keys again.
Pro Tip 2/3Not Literal
'Good grief' isn't about actual grief; it's more of an ironic phrase. 
Good grief, this puzzle is complicated!
Pro Tip 3/3Colloquial Use
It's conversational, suitable for casual talks but not for formal writing or speaking. 
Good grief, the dog's made a mess again.