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1.
A small, disgusting, nugget shaped particle that is often coughed up from ones lung. It smells worse than a rotting corpse and you are scared you are the only one...

You are not alone.
Tommy just coughed up a lung biscuit and it smelled like the breathe of 1000 bums.
by Sir Blump-a-lot February 25, 2010
 
2.
Have you got a lighter? I'm just going pop outside and have a lung biscuit
by barrybuckle September 09, 2009