The Shadowlord said:
This is an expression that bugs me. I always assume it's a massive exaggeration. I can't imagine anyone finding something so funny they literally lose control and throw themselves to the floor and start rolling around like they're on fire. But maybe that's just me, being the cynical pessimist that I am.
It's a metaphor... of course nobody does it, but the symbology shows how funny you think it is...
Think of Monty Python and the Life of Brian, and you will see an entire crowd ROFL... of course that never happenes, but it was a visual representation of how funny they thought the situation was...