Scare, annoy, etc. the hell out of somebody
Definition 1 of 2
Pro Tip 1/3Intensity Emphasis
The phrase exaggerates the feeling to show a very strong reaction. 
When the spider dropped from the ceiling, it scared the hell out of Sara.
Pro Tip 2/3Colloquial Usage
Used mainly in informal situations among friends or in storytelling. 
I love telling ghost stories that scare the hell out of my friends around the campfire.
Pro Tip 3/3Emotional Weight
It adds emotion and color to a story, showing a personal experience. 
His unexpected appearance at the party scared the hell out of me.