1. to laugh.

She was very funny and we cracked up every time she said anything.

2. to become mentally ill.

I lost my appetite, became unable to sleep, and worried that I was cracking up.

• above the law
• a brain box
• a brain drain
• a brain trust
• a breath of fresh air
• a bright spark
• a bright spot
• a broad church
• a Bronx cheer
• Absence makes the heart grow fonder.
• a bucks party
• a bully pulpit
• a bumpy ride
• a bum rap
• a bum steer
• a bundle of joy
• a busman's holiday
• a buzz word
• a call girl
• a calling card
• a camp follower
• a can of worms
• a carbon copy
• a carrot and stick
• a carrot and stick approach