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1.
co2
A molecule containing one Carbon atoms and two Oxygen atoms. Also known as "Duster Gas". It is the gas found inside of compressed air cans (such as the ones for cleaning computer keypads). Also used as a propellant for some paints and liquids.
"Bottles that contain CO2 can explode if exposed to open flame."

"CO2 is compressed oxygen."
by they7692 July 17, 2009
 
2.
co2
Refers to the case when gas pressure built up in the rectum expels feces with an unusual amount of force.
Don't go in there for a few minutes, man, I just took a co2.
by thlayli January 12, 2003