A crack-up is a sudden emotional or mental breakdown where someone loses their composure or ability to function normally. When a student has been studying too hard for too long without rest, barely sleeping and skipping meals, they might experience a crack-up: suddenly bursting into tears over a small setback, or finding they simply can't focus on anything anymore. The term suggests something that has been under too much pressure or strain finally breaking apart, like a wall developing cracks before it crumbles.

The word can also describe a collision or crash, particularly of vehicles. A car accident might be called a crack-up, especially a serious one where vehicles are damaged.

In informal conversation, people sometimes use crack-up to mean something hilarious. If your friend tells a joke that makes everyone laugh uncontrollably, you might say “that was a real crack-up” or “you're such a crack-up.” This usage refers to the idea of someone cracking up with laughter, doubled over and unable to contain their amusement.

The first meaning is a serious one. When someone has a crack-up from too much stress, they may need rest, support, and time to recover.