For Pete’s Sake!
What it means:
This is an expression you would use if you are annoyed or exasperated about something.
When to use it:
Anytime you are annoyed really. There are ruder versions of that idiom (for Heaven’s sake, for F***’s sake…) so this one is pretty safe to use.
Here is how I would use this idiom:
“For Pete’s sake! Why do I always look crazy when I am teaching…I mean just look at my pictures, it looks like I am always pulling a funny face.”