1) Gas from the anus of a human (either living or dead) so bad that is has the ability to choke, seize, or incapacitate another human being.
2) Taking a shit during the day, quite possibly at the office, that is so powerful that it has the ability to overpower the powered air vent in most restrooms and escape into the open air. Note: Zombie gas is not to be confused with ninja gas which has similar properties but lacks the raw power of Zombie Gas.
"wtf did you eat tav? boiled eggs? Your zombie gas is melting my lungs"