When the upper-face or the area around the eyes are shaded or appear as if they are shadowed. Can show disgust, anger, and/or shock. Alternatively, it can indicate violent or sinister intentions. It is used for comedic purposes, and can be either used subtly, when a character is upset and trying to hide it but their expression betrays them, or with exaggeration for comedic effect.
This doesn't refer to cases where a character is in shade or her face is shaded because of a hat/umbrella/other object. (When using object to shade eyes use shading eyes).
