crack up

1.  Suffer an emotional breakdown, become insane, as in He might crack up under the strain. This usage alludes to the result of cracking one's skull; from the early 1600s to crack alone was used in this way. [Slang; early 1900s]
2.  Damage or wreck a vehicle or vessel. For example, I'm always afraid that I'll crack up the car.
3.  Experience a crash, as in We cracked up on the freeway in the middle of the ice storm.
4.  Also, crack someone up. Burst or cause to burst out laughing, as in The audience cracked up, or That joke really cracked me up. [Slang; c. 1940] Also see break up, def. 6. All of these expressions derive from crack in the sense of "break into pieces" or "collapse," a usage dating from the late 1600s. Also see cracked up.

Definition: break down mentally
Antonyms: be calm

The verb has 3 meanings:

Meaning #1: suffer a nervous breakdown
  Synonyms: crack, crock up, break up, collapse

Meaning #2: rhapsodize about

Meaning #3: laugh unrestrainedly
  Synonym: break up