Slang and an abbreviation of the new popular game "Among Us." Instead of saying among us, the streets of The Skeld use this widely popular term to denote the game. The word can also be used for the textbook use of the phrase "among us."
Mold PLUS fungus = Mungus. Instead of saying there's a lot of mold and fungus in the air or on something, there would be a lot of mungus on it.
There is a lot of mungus all over everything in the forest today because it's been a very wet and humid Fall. There's mold everywhere and fungus everywhere. There's even "mungus" in the air!