Definition of Cack

Babylon English Dictionary
(Slang) laugh in an uncontrollable manner; have a bowel movement, defecate; stain with feces
(Slang) hilarious person; feces
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Cack Definition from Language, Idioms & Slang Dictionaries & Glossaries
Webster's Revised Unabridged Dictionary (1913)
(v. i.)
To ease the body by stool; to go to stool.
  
Webster's Revised Unabridged Dictionary (1913), edited by Noah Porter. About
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cack
\cack\ (kăk), v. i. [oe. cakken, fr. l. cacare; akin to gr. kakka^n, and to oir. cacc dung; cf. as. cac.] to ease the body by stool; to go to stool.

Australian Slang
1. (n., v.) cackle; 2. have a laugh; 3. someone with a sense of humour; 4. very funny person: “That guy's such a cack”; 5. faeces; 6. defecate; to soil with excrement: “The baby cacked its nappy” (from Middle English cakken, kakken)
be extremely scared
be extremely scared
laugh uncontrollably
lefthanded
London Slang
faeces, rubbish. eg "What a load of old cack you talk sometimes !".
English Slang Dictionary v1.2
1. rubbish, nonsense
2. excrement
3. See also cack it