Laugh your head off
Definition 1 of 1
Pro Tip 1/3Exaggeration
The phrase is hyperbolic; it does not mean someone's head literally detaches. 
When I watched the movie, I laughed my head off, but of course, I kept my head!
Pro Tip 2/3Informal
Used in casual settings; not appropriate for formal contexts like business meetings. 
At the family reunion, we laughed our heads off at Uncle Joe's stories.
Pro Tip 3/3Emotional Intensity
Indicates strong emotions; use when expressing a deep reaction to something funny. 
His prank was so elaborate, we all laughed our heads off when it was revealed.