Bursting into a prolonged fit of intense, uncontrollable laughter. Instigating this sort of reaction in someone is called "Cracking that person up."
John told me a hilarious joke the other day and I started cracking up.
laughing mad hard. Almost to the point of crying. Laughing you split ends off. more than giggleing or laughing
Guy 1: hey, whats wrong wiff aly?
Guy 2: Oh we where talking about how disgusting Michelle's nose is and then she started cracking up