When the vibe/mood/aura of a room/place seems uneasy and out of the ordinary, the air is heavy and something feels wrong. It commonly foreshadows that
something terrible is about to happen.
The term was heavily popularized in 2017 when the game
DDLC (
Doki Doki Literature Club) was released, and one of the characters (Yuri) uses it.
“Stagnating air is common
foreshadowing that
something terrible is about to happen” -Yuri
2017