It's usually used to describe a look that makes someone feel uncomfortable or threatened. The phrase implies that the person staring is trying to intimidate or challenge the other person.
Here are some examples of how "mean mug" might be used:
* "He was mean mugging me from across the street."
* "Don't mean mug me, I'm not afraid of you."
* "She gave him a mean mug when he cut in line."
The phrase is often associated with gang culture and is considered a sign of disrespect.
It's important to note that the phrase is subjective and can be interpreted differently depending on the context.