swamp gas noun3 •Gases emitted from a swamp.Swamp gas occasionally catches fire. Sometimes lightning sets off a patch of swamp gas. •A skeptical explanation that is provided in order to debunk a UFO sighting that a believer regards as ludicrous, illogical or disingenuous.He explained away an especially peculiar sighting with another helping of good old swamp gas. •(informal) Hydrogen sulfide.
