People say: You crack me up, when someone makes them laugh hard.
As a secondary meaning, it can be used to an emotional or mental breakdown.
“To crack someone up” is an American idiom that originates from the 1600s when women applied so thick make-up that it would crack up and appear flaky if they laughed hard.
According to another theory, the term comes from Gaelic, from the very word “craic” which means “fun”.
Though the term was present in written and spoken language from its appearance in the 17th century, it only started to gain popularity in 1990.
The use of “crack me up” increased rapidly most probably after it could be heard in episodes of the iconic TV series The Fresh Prince of Bel-Air.
Also, a movie was released with the title Crack me up in 1991, which also had a huge part in the spread of the term.
The expression is still commonly used in everyday language.