crack up means "suffer a nervous breakdown". That is the primary sense; the word has 3 in total, listed in full on this page.
What is another word for crack up?
Close synonyms include crack, crock up, break up, collapse, break up. Each sense on this page lists its own synonyms, because a word with several meanings rarely has one substitute that works for all of them.
Is crack up a common word?
crack up is banded "Familiar" — widely understood, used when the subject calls for it. The band comes from how often the word appears in a large corpus of everyday English, not from an editor's impression.
Definitions, synonyms and semantic relations for crack up come from Princeton University’s WordNet. Commonness is measured against a corpus of everyday English. Full sources and licences.