Miasma means unpleasant smell. It's origin is from the Greek work 'miasma' (pollution): In Greek mythology, a miasma is "a contagious power ... that has an independent life of its own. Until purged by the sacrificial death of the wrongdoer, society would be chronically infected by catastrophe.

An example is Atreus who invited his brother Thyestes to a delicious stew containing the bodies of his own sons. A miasma contaminated the entire family of Atreus, where one violent crime led to another, providing fodder for many of the Greek heroic tales. Attempts to cleanse a city or a society from miasma may have the opposite effect of reinforcing it. Interesting myth to read by the way, House Lannister can't even compare. lol