I did not know that yesterday!

A mug shot is "a photograph of a person's face, especially one made for police files." According to Wikipedia, the word "mug" is 18th-century British slang for "face." Wikipedia says, "Another source suggests the term comes from mug, as in grimace, because early subjects would try to reduce their mugshot's value for later identification by grimacing or otherwise twisting their facial muscles (mugging)."

OK, but how did a face come to be known as a "mug"? Over on the Wordsmith.org message boards, one post says: "According to Webb Garrison in " Why You Say It," beer mugs of the late 18th century were often shaped like human heads, and a not especially attractive person often bore a resemblance to a face on a mug. As a result, a face came to be called a mug."

A Word Detective column corroborates this: "It was common in the 17th and 18th centuries to decorate drinking mugs with grotesque caricatures of human faces, and by the early 1700's "mug" had become a popular slang term for "face."

Whatever the origin, we know one thing: Some mug shots are mandatory viewing

Source: ask.yahoo.com