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crack up

B1 informal separable both
In simple words

To start laughing really hard, or to stop working properly in your mind.

Literal meaning: To split or fracture upward — as if pressure from within causes something to break apart.

Meanings

1 A2 idiomatic informal

To burst out laughing uncontrollably.

"She cracked up when he tripped over the garden hose."

Grammar: inseparable
2 B1 idiomatic informal

To cause someone else to laugh uncontrollably.

"His impression of the boss always cracks the whole team up."

Grammar: separable
3 B2 idiomatic informal

To suffer a mental or emotional breakdown under stress or pressure.

"After months of overwork and no sleep, he finally cracked up."

Grammar: inseparable
4 B2 idiomatic informal

Used in the fixed expression 'not all it's cracked up to be', meaning not as good as people say.

"Fame isn't all it's cracked up to be — the loss of privacy is exhausting."

"It's not all it's cracked up to be."

— Common idiomatic expression widely attributed in popular press and interviews, e.g. used by various celebrities discussing fame.
Grammar: inseparable
Usage notes

The 'laughing' sense is very common in everyday speech. The 'mental breakdown' sense is more serious. The phrase 'not all it's cracked up to be' is a fixed idiomatic expression meaning something is not as good as people claim.

Commonly used with

audience class crowd friends everyone tears

Forms

Base
crack up
I/you/we/they
3rd person
cracks up
he/she/it
Past simple
cracked up
yesterday
Past participle
cracked up
have + pp
-ing form
cracking up
continuous

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Synonyms

burst out laughing break down lose it fall apart collapse dissolve into laughter

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