Australian slang.
"Cack" used as a noun means something extremely funny. It is derived from the verb "cack" (as in "cack yourself laughing") but from which of the two meanings I am not sure. "Cack" can mean "die" or "kill" (eg "His heart stopped and he cacked.") probably a derivative of "cark (it)", which in turn probably came from "carcase". As such, to "cack yourself" would, in this case, mean you had laughed yourself to death.
Then again, there is also "cack", a noun, meaning "faeces", or more coarsely, "shit". As a verb this means "defecate". In this sense it is usually applied to people soiling themselves (eg. "The senile old bloke cacked his pants").
Whether it means to die laughing or shit yourseolf laughing is now irrelevant, however. In common Australian usage, the phrase "cack yourself laughing" has evolved into the noun "cack".
I hope this has been enlightening for you all.
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