Pothead slang (from
carburetor) for a small hole found either on the side of a
bowl,
bong, or other
drug paraphernalia. Also sometimes referred to as a
rush hole.
When smoking, you light the marijuana and pull gently with a thumb on the carb. When you are ready to clear the chamber (remove all the smoke from the smoking implement) you remove your thumb from the carb and continue to pull.
This process serves to provide an intermediary state for the smoke between the weed and your lungs. This allows it to cool and provide smoother, but still very potent, hits.
rougevert adds that I "might want to mention that in Australia this is known as a "shoddy", which is also a slang word for a shot of spirit alcohol"