saying

[Dark Angel]

What does the term "shut your face" mean? why face?

[Quizmonster]

This has been a popular, if slangy, term since the late 19th century. It is just an example of the figure of speech known as synecdoche...ie using the part for the whole or the whole for the part. ("He owns 500 head of cattle" doesn't mean he owns only their heads, but also their bodies, hooves, tails etc too.) In this case, the face...ie the whole...is taken to mean the part...the mouth.

[5029]

As for the meaning, it is a derogatory way of telling someone to keep quiet