"Mug" is one of those terms that is rarely used anymore in normal conversation. Nowadays, the most common usage is in "mug shot," which is usually referring to athletes or Hollywood's bad boys, whose mug shots frequently appear on the nightly news. Still, most of us don't need newscast reminders that a mug is a face. And often an ugly one at that!
We've watched the Three Stooges and Humphrey Bogart and Jimmy Cagney films and DVDs that seem to have their own vernacular. This is, of course, just the normal, ever-changing speech patterns, expressions, and nomenclature of each particular generation.
For example, a normal person might say, "I had some competition getting the attention of a woman." But in old-time Movieland language, this is spoken as, "Say, I got into a scrape with a palooka over a dame. A real wise guy. Why, I socked him right in his ugly mug!" (These words are always spoken in a Brooklyn accent, regardless of whether the place is New York, Milwaukee, Chicago, L.A., Mexico City, or ancient Rome. In old-time Movieland, pretty much everybody sounds like Jimmy Cagney or Bugs Bunny.)
"Mug" predates old-time Movieland, though. It has been used as a noun to describe the face and as a verb of the word "grimace" since the 17th century. It most likely derives from "mugg," a Scandinavian word for a drinking vessel.
In the 17th and 18th centuries, mugs were often decorated with cartoonishly-drawn human faces. This may have led to the use of "mug" as a synonym for an ugly face. Although a bit archaic, "mug" is still used to describe a less-than-beautiful face.
In the modern world, one would ordinarily talk about Mickey Rourke's mug or Keith Richards' mug, as opposed to Jennifer Anniston's or Brad Pitt's mug.
Keith Richards (left) and Brad Pitt (right). Can you tell the difference?
Another theory is that "mug" comes from "mukha," the Sanskit word for face. They have yet to determine, however, whether Sanskit is ever spoken with a Brookyn accent.
The term "mug shot," referring to a photo taken of a prisoner or someone in police custody in a jail or prison, is thought to haves derived from the "grimace" definition cited earlier. A person in police custody would after "mug," or grimace in order to change the shape or features of his face. This was done to make him less easily identifiable.
Arrestees "mugging" for the camera.
One last "mug" usage is, of course, the noun (often called a "mugging") or verb, the act or action of assaulting someone, almost always to rob them of money. This probably (I cannot locate its origin anywhere) is in referring to the perpetrator of the assault/robbery crime as a "mug," or an ugly-faced criminal.