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.
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!