There are 2 definitions of crack up in English:

crack up1

1 informal Suffer an emotional breakdown under pressure: I feel I’m cracking up, always on the verge of tears
More example sentences
  • I was short-tempered and I was cracking up a bit under the pressure of it all.
  • ‘If I had not had Diana and the children I would have cracked up and had a mental breakdown by now,’ he said.
  • His stylish arthouse flick looks at three women as they crack up.
Synonyms
break down, have a breakdown, lose control, be overcome, collapse, go to pieces, go out of one's mind, go mad, crumble, disintegrate
2 informal Burst into laughter: she tries to keep a straight face, but she keeps cracking up
More example sentences
  • He cracks up into laughter, his shoulders heaving uncontrollably.
  • I really cracked up in laughter when I read the headline ‘Protest is the backbone of democracy’.
  • Before I knew it I was buzzed and cracking up with laughter.
Synonyms
burst out laughing, dissolve into laughter, roar with laughter, shake with laughter, laugh uncontrollably, guffaw, be doubled up, split one's sides, hold one's sides
informal fall about, be in stitches, break up, crease up, be creased up, be rolling in the aisles, laugh like a drain
See parent entry: crack

Definition of crack up in:

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Word of the day dinkum
Pronunciation: ˈdɪŋkəm
adjective
(of an article or person) genuine, honest, true

There are 2 definitions of crack up in English:

crack-up2

Line breaks: crack-up

noun

[usually in singular] informal
1An emotional breakdown: he had a complete mental crack-up
More example sentences
  • Mark has had one emotional crack-up and one hospitalisation before the stresses have even begun.
  • The directorial debut shows the actress grappling with the aftermath of a crack-up.
  • By the time the reader gets to book four, Donny's been ‘hospitalized for a crack-up.’
2An act of breaking up or splitting apart: the crack-up of the Soviet Union
More example sentences
  • It would lead to the crack-up boom and the breakdown of the whole monetary system.
  • Any bump in the road is greeted not just as evidence of the crack-up, but as its beginning.
  • The later the crack-up comes, the longer the period in which the calculation is misguided by fiduciary media.

Definition of crack up in: