Laugh, scream, etc. your head off
Definition 1 of 1
Pro Tip 1/3Exaggeration
The phrase uses hyperbole and doesn't mean literally losing one's head. 
At the party, we laughed our heads off at the absurd costumes.
Pro Tip 2/3Intensity
This phrase emphasizes a very high degree of emotion. 
She was screaming her head off when her favorite band came onstage.
Pro Tip 3/3Informal
It's casual language, appropriate for friendly conversations, not formal ones. 
They were crying their heads off after watching the funny movie.