Fall about
Definition 1 of 2
Pro Tip 1/3Humorous Scenes
Used when describing situations where something is so funny it causes people to laugh uncontrollably. 
The comedian was so hilarious that the audience fell about.
Pro Tip 2/3Group Laughter
Often used when everyone in a group starts laughing together. 
At the family reunion, we all fell about at Uncle Joe's stories.
Pro Tip 3/3Exaggerated Reaction
Can imply that the reaction is more dramatic or exaggerated than usual. 
She heard the punchline and fell about as if she'd never stop laughing.