Noun. An aggravated or angry facial expression.
A prolonged, directed mean mug is often enough either start or avoid physical altercation.
Verb. To maintain an angry facial expression.
Three 6 Mafia use mean mug adjectively: Mean mug niggas lookin and they hid from a smile...
Baby Bash uses mean mug as a noun: Y'all know whats up / Look on my face and its called the mean mug.
Xzibit (feat. Keri Hilson) as a verb: All the girls mean muggin / cuz they musta seen what a girl pulled up in