fall about laughing

To be consumed by uncontrollable laughter. Primarily heard in UK. We all fell about laughing when the professor accidentally swore during his lecture. My brother-in-law's wicked sense of humor has us falling about laughing every time we hang out with him. Once one actor falls about laughing, it's only a matter of time until their scene partners join them.

fall about (laughing/with laughter)

(British English, informal) laugh a lot: When he watches Charlie Chaplin films, he falls about laughing. I don’t find them funny at all!