What in the heck?

Meaning

A very common, mild way to show annoyance or surprise. It is the go-to phrase for people who want to sound frustrated without actually swearing.

Mild, approachable

Used in a sentence

• What in the heck are you doing up at three in the morning?
• What in the heck happened to my sandwich?
• I looked in the box and it was empty. What in the heck?