Definition - What does Third Lung mean?
In cannabis culture, third lung is a slang term for a large smoking apparatus used for cannabis. It is easily made at home and holds large amounts of smoke. Other names for a third lung are reverse gravity bong, iron lung, rat's tail, lungie, anti-gravity bong, and parachute.
The term third lung is also a brand name for scuba diving equipment. Contrary to what it may suggest, the term third lung does not denote a person who takes exceptionally large tokes from regular joints.
MaximumYield explains Third Lung
The most common construction materials of a third lung are a soda bottle. You will also need a slide or bowl. If you don’t have that, then you can also use tin foil. Additional materials include a knife, a plastic bag, duct tape, and a small piece of string or cord. Making a third lung is simple and produces an apparatus that holds a lot of smoke at once, which when inhaled will result in a powerful high.
Start by cutting off about 4.5 inches off the bottom of a 2L soda bottle. Create a hole by burning that is large enough to fit the slide, bowl or homemade tin foil bowl. Then, take the plastic bag and secure it around where you cut off the bottom. Tape it around this area, making sure the seal is tight up around 2 to 4 inches around the bag. You can optionally tie the string around the bottom with which you can pull the bag when you are inhaling.
The third lung is best used with more than one person. First, push the bag as far in the bottle as it will go. Then, light the bowl. Pull the bag down at a steady pace to create a vacuum to accumulate smoke. Then, pull out the slide and clear the bottle.