noun

• (informal) A cackling goose (Branta hutchinsii)
  "[…] cacks, " the whitefronts or specklebellies in the Sacramento Valley"

verb

• (of a bird) To squawk.
  "Still fluffy with down, she often attacks the other birds, cacking and flashing her wings, or threatens me as I watch through the tiny peephole of the near box."
• (brass instrument technique) To incorrectly play a note by hitting a partial other than the one intended.
  "The bugler hopes not to cack during his performance."

verb

• (intransitive) To defecate.
  "Can't cack... won't cack, My bum hole has gone on strike, I've huffed and puffed, grunted and groaned, And squeezed as hard as I liked, I've leant myself backwards, leant to the front, Raised my knees and then put them back, I've gripped the seat and pushed like hell, But still...I can't have a cack."
• (transitive) To defecate (on); to shit.
  "‘I asked him once if he got nervous before doing it,’ says Astin, ‘and he said he was absolutely cacking himself before going on stage, but as soon as he got there it was fantastic.’"
• (transitive) To excrete (something) by defecation.
  "He smelled the ferrous oxide of blood and the farmyard stench of shit. He'd cacked it. He was empty and he'd cacked his load. The brushman came over, lisping slightly[…]"
• (US, slang) To kill.
  "He tried to shoot me, so I cacked him."

noun

• (slang) Excrement.
  "I dive back in along the cobbles, step over the cack, wall an fence closin me down, barbed wire left, razor wire right, follow by puttied-in broken glass, spike rails an stuck-on warnings about dogs."
• (slang) Rubbish; anything worthless.
  "'See that' said the man, pointing with his fork at a trace of soemthing on the rim, 'that's not been washed properly. That's a bit of old cack on there.'"