- (idiomatic, intransitive) To laugh heartily.
- It was hilarious. We were cracking up the whole time.
- (idiomatic) To cause to laugh heartily
- The joke about the nuns in the bath cracked me up.
- (intransitive, idiomatic) To become insane; to suffer a mental breakdown.
- She got through the war, but cracked up when her sister died.